I don´t understand how to interperet the evaluation in chess.com analyser

Can someone please help me understand.

Move 11 (black): Queen f8 = -3.71 (best move)

Move 12 (white): Queen d1 = -1.02 (best move)

Move 12 (black): Pawn h6 = -3.90 (best move)

I don´t get it. How can the evaluation increase 3 (white becomes 3 units better) when white makes move 12? Shouldn´t this move be considered at the previous evaluation of blacks move 11?

And same thing with blacks move 12,  how can the evaluation decreese (improve for black) almost 3 units?

OK I would understand if the evaluation may increase some 0.1 units or so because evaluation on the next move looks one move deeper. But this I don´t understand.

Queen f8 = -3.71:

Queen d1 = -1.02:

Pawn h6 = -3.9:

I tried increase analysis depth 18 -> 22 and now I don´t get this strange result.  Move 12 for white is evaluated as -4.09 (instead of -1.02). So it was the evaluation depth after all. But I never thought it would make such a big difference. 

It seems you have encountered the horizon effect.

https://en.wikipedia.org/wiki/Horizon_effect

It was more pronounced in the early days of computer chess, when search depths were much more limited by processor speed than they are today.

I see now that you can set evaluation depth higher than 22 as a diamond member. What would be the approximate equivalent ELO for the chess.com engine (Stockfish?) with the evalustion depth set to 18 or 22? Not that I think it would really matter for me (I'm 1050) but just out of curiosity. 

#1
"Move 11 (black): Queen f8 = -3.71 (best move)
Move 12 (white): Queen d1 = -1.02 (best move)
Move 12 (black): Pawn h6 = -3.90 (best move)"
It makes no difference: -3.71 = -1.02 = -3.90 = black is winning

Yeah no real difference except when I was analysing my game move by move I was like "how did all of a sudden my 4 point lead drop down to 1 point when I only made best moves"?