Fair enough. I imagine as a PhD its easier to avoid since you're doing new research, so you're presenting unique information with (in theory) unique sentences.

Whereas for a lot of undergrad students, up until the tail end of their degrees, they're writing about fairly extensively covered topics, so you're much more likely to accidentally steal wordings from others who have already written about them.
In fact at that stage, I'd bet having too low a plagiarism score would more likely indicate you're barking up the wrong tree.