There are several reasons why certain icons/logos are the way they are.
Copyright: Ask Different is not allowed to use Apple's own logo. They settled for the Mac's Command Key logo instead, which was already 'public domain'.
Size: I personally like the Mathematics logo; when it's in full size you can see it's derived from the Reutersvärd / Penrose triangle. ...
Stack Overflow, as the first site on the network, always had sans-serif, I think. It's pretty common for websites. Sans-serif is essentially the default for Stack Exchange sites.
The math site did not start out with serifs. This changed when the site got a design because Jin, the designer at the time, "believe[d] the serif typeface on the header and ...
Some sites decided that serif fonts were more appropriate for their subject, so they have our serif font stack rather than the default sans serif stack.
There's more information on this answer about our fonts but I haven't checked whether the exceptions are still correct (I don't think they are).