100%, to me, means the main game. Like, story completion and map discovery. Challenge chevos are for tryhards like myself that want a little extra pizzazz on top.

Let me try a metaphor. Say you've ordered some kinda cocktail from a bar. A no-fuss bartender pours your drink and is done, but a "mixologist" does crazy bottle flips and weird knife play involving fruit. Either way you get 100% of your drink (the game), but one looks a bit fancier if you're into that kinda thing.