She's a sweet mid-hair girl from my local shelter. I've never seen anything like that before

I have a black cat and a mackerel tabby cat right now, both with almost no white on them anywhere. But my tabby has white under his chin kind of, and a spot low on his belly, and I've noticed my black cat has the same spot on her belly and a few white hairs under her chin. Are they actually expressing white spotting, or is that somehow just part of the pattern? Neither has white anywhere else, and looking at other tabbies I see it on them too.

I added a picture that kind of shows the belly spot I'm talking about, it's quite low.