Well, if it's up against a far superior engine, it may be losing well before it realises it. So, say in some position the stronger engine plays a move (the best move) that Crafty hadn't considered as dangerous, it may evaluate that as an error, only for the eval to whipsaw against it when it finally sees what's going on.
That's just once scenario, there are certainly others, for example it not having tablebase access (I don't know if it does) and it's opponent does - can certainly cause some very weird disparity in their respective evals.
I noticed some of the engines are having drastic eval changes and wanted to know why it's happing. I've seen it the most severe with crafty which goes to a drawn position to a actually eval every other move sometimes. I've been asked about this as well in the chat.
-ENcyndaquil-pikachu/systiem a volunteer moderator