I’ve just returned from a drive to Washington and back. I used cruise control as much as possible. I noticed the same thing. I’ll be cruising at 65 or 70 mph and someone will pass me and then move into my lane (right lane). Then they would slow down. Still on cruise control, I would have to pass them. As I pull alongside, they would match my speed. Weird.
On these boards I have referred to “left seat passengers” or “left seat zombies”. These are people who are not driving the car, but are merely riding along and they happen to have a steering wheel in front of them. As a pilot, I was taught to “Fly the airplane!” (When I learned to fly, it was before everyone used headsets so the instructor would have to shout.) Because of this, it bothers me when I’m forced to be around people who will not “Drive the car!” Left seat passengers bug the hell out of me.
I think what’s happening is that they don’t use their speedometers as a reference, but depend on their positions in relation to other vehicles to gauge their speed. If someone is overtaking another vehicle, they suddenly say, “Oops! I’m speeding! I’d better slow down, lest I get a ticket!” Then after they’ve slowed down in front of you and you’re passing them (on cruise control) they say, “Hey! I just passed this guy! Let me look at my speedometer. Hm. I’d better speed up!”
In Driver’s Ed. in high school the instructor called some people “packet drivers” because they tended to gather together in packets. ParentalAdvisory suggests (perhaps in jest) that they “want the company of other people”. I think that they are just Left Seat Passengers who are just not paying attention until they have external reference points like other drivers (who are often also LSPs).