Why is this a missed win? I found myself in this position as Black. I played Kb6, but the engine on chess.com suggested that the move Kb7  is a missed win, and the continuation below would have been winning for me. It gave no lines beyond what is below, and I can't see how this is a win for Black. Is this a fault with the engine, or is there some line that I've completely missed?

[Title "Unexplained"]
[FEN "2b2r2/2p5/p1kb2qp/1p1p4/3P4/7P/PP1BQPP1/RNR3K1 b - - 0 1"]

1...Kb7 2.Qe3 Rg8


In response to comments questioning if I have the position wrong or missed a root, I confirm I was a full rook down and felt this was a completely lost position. I have now included a screenshot demonstrating the strange opinion of the engine at chess.com

I guess it's a bug with the engine then?
This is not a missed win for black, both Kb6 and Kb7 are equally losing for black as you are simply a full rook down without any real compensation. Qe3 move simply stops black's only way to breakthrough as it stops Bxh3. That is not white's only move either, Kh1 and Rc3 (though walks in the way of the knight) are sufficiently good for white too.

So either: 


The cloud engine didn't get to run sufficiently long and gave a wrong evaluation, or
you simply mis-read or mis-interpreted the evaluation of the engine, or 
you're showing us the wrong position here.