I don't think this is adaptive speed control (ie radar cruise control) as that, at least on our Toyota that's my only familiarity (and thus a globally common experience no doubt haha) it ignores cars in the adjacent lane and will pass right by them.
No, this is the driver consciously or unconsciously matching your speed. I've noticed what I call a "rubber band" effect where if I try to pass someone at a small speed differential (like I'm going 65 and they're going 60), they'll speed up and pretty much match my speed, and when I slow down they slow down... I don't think it's conscious for the most part, it's a form of fixation.
If I speed up a bit, they match, I speed a bit more, they match, until they realize we're going 80 or 90 and the rubber band breaks and they drop off. Any more when I pass people I try to do it sufficiently quickly that they have to be thinking about it to match the speed; if I catch a rubber band, I'll reduce speed gradually and then suddenly race forward before they react.