When different chess engines play against each other, they don’t always have the same outcome. Why not?

Question

This has bugged me for a bit. If chess engines are meant to always play the best moves, then how come two chess engines playing against each other doesn’t always have the same winner?

In: Other

Answer 1

Some engines do not work deterministically and may use heuristics (“guessing”) to figure out which move is better without fully calculating it.

This is without discussing other rules, like human prior moves.