Why is "?" a mistake but "!?" an inaccuracy? Seems like vice versa would make more sense.
There's a whole set of annotation symbols that were used on the Informants to make comments without requiring the use of language.
https://en.m.wikipedia.org/wiki/Chess_annotation_symbols
Here is the definition of "!?" :
So, I guess the answer is that in chess, ! means a good move or idea and that ? means that it's not a good move or idea. To clarify, an inaccuracy is actually represented by ?!, with the question mark first. This basically means that it's a questionable move, as the question mark comes first, but there could possible be some small merit to the move. A mistake (?) says, "That's not a good move at all."