I've just played a game on Chess.com, and I'm surprised that the engine considers a promotion that I made a huge blunder. Like, my position was rated -62.8 (basically a won game), and after I promoted my pawn to a queen the rating dropped to -6.40. That's more than 50 points with a single move!
This is the position:
[FEN "8/8/2B5/p7/8/p4p2/P2K1kp1/8 w - - 0 1"] 1. Be4 g1=Q (1... Kg3 2. Kd3 g1=Q 3. Kc4 f2 4. Bd3 f1=Q 5. Bxf1 Qxf1) 2. Bxf3 Kxf3 3. Kc2 Qc5+ 4. Kd3 Qb5+ 5. Kd2 Qb2+ 6. Kd1 Qxa2 7. Kc1 Qb2+ 8. Kd1 a2 9. Ke1 a1=Q# 0-1
I played g1=Q, but the engine says I should have played Kg3 instead, which would have kept the rating at -62.8. I don't understand why. My pawn in f3 was threatened by the bishop, but it was protected by the king. The white king was unable to come closer, because of my king. And the pawns on the left were irrelevant.
The engine's move did lead to another pawn being promoted... Only to be captured by the bishop, which in turn was taken by the first queen. So after a few moves the situation was identical: king and queen vs king (plus the pawns on the side). And even with my "blunder", winning was easy.
Why does the engine find such a huge difference? Should I just ignore this, or am I missing something important?