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

There isn’t only one best move, especially in the opening.

For the first five to six moves, there are multiple options called “book moves”, which are all equally likely to bring to victory. After that, the possible best moves reduce to one or two, only to increase again in the late game.