My hobby is to try to differentiate between professionals and amateurs in the "water trade."
A mysterious person is really really good at what they do. So good that they aren't exactly sure what is it they do, but you know they're definitely an expert at whatever it is.
This reading for this word is mysteriously an exception (sorry). So here's a mnemonic to help:
If you're such an expert you should be able to tell what anything is if someone asks you. Even this crow toe (くろうと). Crow toes are pretty hard to recognize, unless you're a professional crow guy. But I guess you are, huh?