Playing stockfish's best move only increases the length of the forced mate

lichess.org/analysis/2k5/8/8/1BKN4/8/8/8/8_w_-_-_0_1#0

Stockfish says this is a #10

playing kc6, according to stockfish, makes it a #11

however kd6, reduces it to #9

If you let it analyze for long enough after 1. Kc6, then it becomes #9. I found it to become #9 at around depth 70, at the latest.

@Creepy_Raven Thank you, I now realize what I should have known all along: that the depth obviously doesn't mean it has calculated all possible moves until that depth/2. I thought that at a depth of 36, it would had calculated 18 moves (36 ply) ahead, which would have allowed it to calculate the #9, but no.