My underlying question here is: What is depth?
I know depth is supposedly the number of half-moves or plies from the starting position that it has calculated, in at least one line. But I feel this explanation is not congruent with the result you're seeing here:
- I'd expect mate in 7 to be found at depth 14, but I understand it didn't search every line so okay it found it when it had already searched somewhere to depth 21. So far so good.
- But, now it finds M13, or mate in 25/26 ply, at depth 21, where it should be at at least depth 25. How?
- Furthermore, the depth keeps increasing after finding mate in 13 (ply 26). Why would it keep searching in sequences that are longer than the mate it already found? All subsequent analyses should stay below 26 ply right?