Ah, sounds like a quiz/exam question. If you take a simple PFC corrector, it has inherent reactive components, it has to out of necessity, so at min load it is more optimum ,"tuned," so to speak. So the simple answer since I don't know which PFC device you refer to is that PCF correctors have a range whereby they optimize, sort of like a tuned circuit. When you are out of the ,"Tuned," range the power factor gets worse. Also I should mention to make a PFC for heavy loads requires more expensive parts to deal with the current, so you could say, a rip off, cheats. Although one should carefully check the ratings and where it indicates a certain power factor. It either meets it's stated advertising or it's garbage.
Without getting into the math, which is really all it is, you probably may not get, I'll just leave it at that. Hope it helps.
