why isn't even engine-to-engine match ended up a draw?

And why does stockfish sometimes lose?

I think if the engine that finds its own best move and another engine that finds its own best move, fight against each other, it will end in a draw.

I looked at the database for engine-engine matches. There were a few game histories, where stockfish lost(not draw!) to relatively weaker ones(still extremely strong in the world though). even though their elo difference aren't tiny and there are no any handicaps. this was quite surprising for me.