I prefer to think of the classes as a guideline, similar to if somebody introduced themselves as an engineer at a party. You'd get visions conjured up of them with a spanner going around fixing things, and you might be right. But you're also just as liable to be wrong.
If an NPC is described as roguish (and we're not talking about their looks!) they are more likely to be stealthy than not, more likely to be capable of high levels of sneak attack than not but it's not set in stone. A Ranger might turn out to be someone with ranger levels, or they could simply be good with a bow and maybe survival but lack the rest of the focus, such as spells or pets.
I find talking about NPCs in terms of class levels can make it easier to point out their demeanour than trying to simply describe it, especially when dealing with people from multiple countries.