Why are some positions +200 instead of M10 ?

Some positions are evaluated as +200 instead of forced mate. Is it because the computer still sees no mate (although I let it run for a while and it's the same thing)?

Also, here's a stupid question :

The eval is +10 for white. Let's say both players play all the best moves. White will win, right? So it's technically possible for White to force a win. So why doesn't the eval say there's a forced mate (I agree it's a strange idea)? It can't be because white still can blunder the game, because when there's a king vs. king and queen endgame, the evaluation says there's a forced mate - when you technically can draw by stalemate, agreement, if the clock of the player with the queen hits zero (considered a draw) or insufficient material, and the player with the lone king can win by resignation. I know this is weird, but I was just wondering.

Horizon effect - Wikipedia

Chess engines can only calculate to a certain depth. If an engine is set to 20 depth, it will look 20 moves ahead and use the final board states of the final calculated depth to determine the best move to make.

So, if an engine is set to 20 depth, and there's a mate-in-25, the engine isn't going to see it.

Nicely done

True except that the look-ahead depths are expressed in ply (half-moves), not in moves.

So an engine set for 20 ply would miss a mate in 11 moves (since that's 21 ply).

Oh, that's good to know, haven't heard that term "ply" before.