How can a Perceptual Rendering Intent Preview still have Out of Gamut Colours?

I've been experimenting with different rendering intents recently, and noticed that with some of my files the out-of-gamut warning areas vary between different intents. (I've shown perceptual and absolute in the examples, but the warning areas are different in relative and saturation too. My warning colour is green.)

It's easy to understand the out-of-gamut warning for absolute colourmetric: it's the areas which cannot be rendered in the printer's colour profile. But what does the warning areas indicate in perceptual preview mode? Surely the point of the perceptual rendering intent is to bring all of the colours in-gamut, which would mean there should be no out-of-gamut colours?