How are the openings classified? As in, what does the 'A' stand for in A27 or the '16' in C16?

A00 [Start position] 
A10 [English]  1.c4
A20 [English: King's (1...e5)]  1.c4 e5
A30 [English: Symmetrical]  1.c4 c5
A40 [Queen's Pawn Game]  1.d4
A50 [Indian: 2.c4]  1.d4 Nf6 2.c4
A60 [Benoni: 3.d5 e6]  1.d4 Nf6 2.c4 c5 3.d5 e6
A70 [Benoni: Classical]  1.d4 Nf6 2.c4 c5 3.d5 e6 4.Nc3 exd5 5.cxd5 d6 6.e4 g6 7.Nf3
A80 [Dutch]  1.d4 f5
A90 [Dutch: 2.c4 Nf6 3.g3 e6]  1.d4 f5 2.c4 Nf6 3.g3 e6

Seems pretty random, B's and C's are kingpawn games

The only thing I know is the variations of one opening usually have the same letter... Otherwise I barely see logic in it.

Even I don't see the logic in it...

Btw.. A00 is the Ware opening AND the Amar opening, not the starting position, and both are completely different openings.

ECO openings is a system which is used to go systematical through the openings.

For that, we used A-E, to classify the starting moves. The 1-99 after the letter is where it is in the system. A big opening, lets say ruy lopez (1: e4 e5 2: Nf3 Nc6 3: Bb5) got alot of numbers, because it is a big opening. A lesser opening got less numbers because that there is not so many sidelines.

A is:

Openings not starting with 1: e4 or 1: d4 (english, birds and so on) (A00-39)

1: d4 without by 1: .. d5 or 1: .. Nf6. (A40-44)

1: d4 Nf6 without 2: c4 (A45-49)

1: d4 Nf6 2: c4 without g6 or e6. (A50-79)

1: d4 f5 (A80-99)

B is:

1: e4 without 1: e6, e5 or c5 (B0-19)

1: e4 c5 (B20-99) (A classic example of extremely big opening)

C is:

1: e4 e6 (C0-19)

1: e4 e5 (C20-99)

D is:

1: d4 d5 (D0-69)

1: d4 Nf6 2: c4 g6 with d5 grünfeld (D70-99)

E is:

1: d4 Nf6 2: c4 e6 (E0-59)

1: d4 Nf6 2: c4 g6 (without d5) (E60-99)

That's rather complicated, and not easy to understand at all...

Why are there so many variations anyways? And how do they arise?