Opera has a classification system with voice types like tenor, soprano, etc., the vocal range is around two octaves for all of them.
But that's just a classification system for opera. It's there to simplify things for composers ("assume that these voice types with such-and-such range is available") and for people hiring performers ("You can sing C4 to C6? You're a soprano. Next!"). Pop/rock operates under different constraints, so the opera system is less useful there.
Invididual singers vary. As described in the wikipedia link, if someone has a vocal range that covers more than one voice type they would get assigned a type based on where they sound best or can sing most comfortably.
Two octaves of usable range is indeed pretty common. For untrained singers it can be less than that (although most people can do at least an octave to an octave and a half), while a few singers have exceptional ranges of 4-5-6 octaves (e.g. Mariah Cary, Morten Harket from a-ha, Dimash Kudaibergen).
So the labels are approximate, yes. But it makes sense if you think of them as labels for the roles within an opera, rather than for the voices of individual singers.