Understanding how analysis engine works

This is a rare good game that I played.
I am not interested in the game itself, but I want to better understand how the analysis board's software works.

On move 17-18 it suggests taking the bishop would lead in a #15 moves forced checkmate. I checked with the queen and, in that case, the engine shows a #9 moves forced mate. How is it possible that my move is "better" than the one suggested by the software? Shouldn't the engine calculate the most effecient line?

I didn't push the analysis depth, but it seems strange to me that an engine chose a worst move compared to the mine. Plus, the next move (my opponent king), it suggests a #11 moves mate...

Hope you can explain, thank you!

The engine is not perfect. That simple.

The analysis board is running on a device you own and it takes a while to calculate the fastest mate.

I'm not sure why you find that at all surprising.

On my computer it takes 0.4 seconds to find the shortest mate which it finds at depth 35 after evaluating 5.6 million positions.

Before 0.4 seconds it reports that Rxc5+ is a mate in 11.

Chess engines build a zero-sum game tree for some given position ... in order for these tree-building algorithms to produce useful results in reasonable time spans some degree of 'hueristic pruning' of the tree must occur. Otherwise the deeper the search goes the number of new nodes to calculate becomes astronomical very quickly. Even a super fast machine doesn't have enough CPU to churn through a gigantic unpruned game tree in what is called 'reasonable time' ...

Hueristics are never perfect, or even particularly 'scientific' ... sometimes the best line ( branch ) gets pruned in the bud long before the numbers start to look better ...

Hope this helps.

Got it, thanks!