Why do elementary leapers contribute similar value to a piece when they have different mobility?

Most people who have looked into it suggest that adding a Wazir (0,1 leaper) and an alfil (2,2 leaper) contribute roughly the same value to a piece containing them? As does adding a Dababba (0,2 leaper), or adding a Ferz (1,1 leaper). 

Intuitively, you're adding four possible moves in all cases, but on a finite board, the average mobility ranges from 2.25 to 3.50. 

I also see no reason not to include the Threeleaper (0,3 leaper) or any of the symmetrical types of half knights in this (half 1,2 leaper). Seeing as they add more mobility than an alfil. 

As full pieces, it is intuitive that several have deep flaws that aren't related to their mobility values but affect practical use. The Alfil is colorbound to 1/8th of all squares. The Wazir is so slow that over 70% of the board is at least 4 moves away from a random position, and most of the time pieces start on one side of the board opposite their opponent, not randomly distributed. Nearly the worst case scenario for slow pieces. The threeleaper is even more colorbound than the alfil, with a geometrically longer move. Some of the radially-symmetric half knights are secretly colorbound to a fifth of the board, the more normal ones are stuck to a fourth of the board, etc. Many elementary leapers do not make for great pieces on their own. 

But these disadvantages usually disappear when elementary pieces are combined. Combination pieces usually come with greater maneuverability starting two moves down the line and continuing from there, often lose their colorboundness (in the Knight's case, combining effectively colorbound pieces makes a non-colorbound piece), and have long enough moves to traverse the board reasonably quickly in most cases. 

Right, so if we think of all of the four-directional leapers that have both right/left and up/down symmetry, because rotationally or completely asymmetrical leapers are gonna either be unbalanced or very annoying (merging a diagonal or clockwise or counterclockwise half-knight into another piece sounds very annoying to play with unless you're an engine, and merging a crab into another piece is blatantly overpowered), we get the following list: 

Wazir, Ferz, Dababba, wide (rl) half kNight, narrow (fb) half kNight, Alfil, tHreeleaper. 

So according to received wisdom we can combine these *mostly* however we want to get knightish-strength pieces. Right, here is every combination: 

WF - king, 6.5625 mobility.

WD - woody rook, war machine, 6.5 mobility.

hNW - unnamed? 6.125 mobility.

vNW - unnamed? 6.125 mobility. 

WA- waffle, 5.75 mobility. 

WH - unnamed? 6.0 mobility. 

FD - Kirin, 6.0625 mobility. Colorbound.

hNF - unnamed? 5.6875 mobility.

vNF - Fibnif, 5.6875 mobility.

FA - Elephant, 5.3125 mobility. Colorbound.

FH - Frog, 5.5625 mobility.

DhN - unnamed? 5.6125 mobility. Effectively colorbound.

DvN - unnamed? 5.6125 mobility. Effectively colorbound.

DA - Alibaba, 5.25 mobility. Doubly colorbound.

DH - unnamed? 5.5 mobility.

N - Knight, 5.25 mobility.

hNA - unnamed? 4.875 mobility. Effectively colorbound.

hNH - unnamed? 5.125 mobility.

vNA - unnamed? 4.875 mobility. Effectively colorbound.

vNH - unnamed? 5.125 mobility.

AH - unnamed? 4.75 mobility.

Now, there are some of these pieces widely acknowledged to be clearly weaker than a knight. I will go ahead and say that an Alibaba or Elephant are clearly inferior to a knight. As are all of the pieces stuck on even/odd ranks or files, and the Kirin. Some people say the king is worth considerably more than a knight. Yet the WD is considered equal despite being more maneuverable and faster across the board than a king.

Observation 1: the knight is not very high on the list. Knights must be pretty good at being knights to overcome that level of mobility disadvantage.

Observation 2: The actual range of mobility here is quite dramatic. The king and WD are around 37% more mobile than the AH.

Observation 3: There are some pieces people really use a lot and some they don't. DH and WH have high enough mobility but are rarely if ever used. Meanwhile, some of these things have been invented repeatedly.

Observation 4: the strongest pieces in pure mobility terms trade maneuverability for mobility and so having two types of relatively short moves together seems to hurt mobility. A king can reach 16 new squares on their second move. A WD can reach 24. Whereas a knight and most of the other low mobility but decently strong pieces can reach 32.

Observation 5: WH can reach 28 squares on its second move making it relatively unmaneuverable.

Observation 6: I'm not sure if we should be talking about maneuverability "on an infinite board" or maneuverability on the finite board or somewhere in between. But I think finite board maneuverability does have some decent arguments going for it. The math, however, is complicated, as there are cases where you could reach a square on move 2 by departing from the board on move 1.

Observation 7: WvN is not very maneuverable reaching 22 new squares on move 2. Fibnif is better but still not amazing, reaching 26. Many of the colorbound pieces also have substantially lower maneuverability than a knight.

Observation 8: there are no symmetrical pieces here that reach more squares in 2 moves than the knight. If there's an advantage to things like the AH or vNH, it would have to be further down the line, something to do with how quickly they traverse the board, or what the opponent's position looks like (pieces with H are good at sniping enemy pieces over a row of pawns for example).

So the question is, what makes it worth trading many of these pieces for a knight, and more importantly, what makes symmetrically adding 4 new move/captures within a radius of 3 or less a reasonably balanced thing regardless of where the captures actually are? Or is it?