I am not talking about the monsters and villains the characters will be fighting, though some of my points could apply to them as well.
Back in the day the level of an NPC was static. The level was determined by the location of where they were from and what seemed most appropriate. Guardsmen in a city were a higher level than those for a village. It made more sense that way, as a city had a higher number of available guardsmen so logically it made sense those of a higher level would be made guardsmen. In addition, guardsmen in a city had to deal with higher level threats and thus could be expected to be of a level high enough to deal with them (excluding the threats only a PC hero could handle).