Is sugar in mead different from "regular sugar"?
Honey usually contains mainly naturally occuring glucose + fructose, with ratio typically 100:104-170. Sacharose occurs as well, but in minority.

Fermentation consumes just part of sugars, but that is not limited to mead. It applies to beer,wine, and bases for spirits as well.

E.g. typical "future beer" before fermentation has sugar content  $\pu{100-120 g/L}$.