This can mean that in sewers as level 1 character you encounter rats that are tough to beat (as stereotypical RPGs have), but when you return there with level 20 character, the rats are either also level 20 and about as hard to beat as they used to be, or are replaced with an opponent that matches your level 20, such as a greater demon. It could also mean that the thieves that rob you on a highway are tough but reasonably thief like characters, but when you reach level 20, they're equipped like epic heroes (the kind you're likely supposed to be).

There is no real realistic way to use level scaling, so it will seem strange no matter how you do it. The least unreal solution is to limit use of level scaling to a very few locations.