Things can get a little more complicated, especially under fatigue where motor unit cycling1 comes into play, but that's the
basic gist of where this idea comes from: Type 1 muscle fibers are recruited first and take a long time to fatigue, leading you to think they'd grow the most when exposed to lighter weights for high reps. Type 2 muscle fibers are recruited more when the muscles are loaded heavier, at least for the first few reps, leading you to think they'd grow the most when exposed to heavier loads for lower reps. 2