I'm probably way off, but think of this:

You are at a certain position behind the car in front of you. (1) If you get closer, you get into the "slipstream" from the car in front of you and your air resistance diminishes, because of slipstream. Hence your speed increases due to constant throttle. Your cruise control adjusts throttle to slow down. Your distance to the car in front of you increases. You get out of slipstream. Air resistance goes up, speed goes down, throttle is adjusted, you get closer to the car in front of you. You get into the slipstream, and from here we continue at (1)...

If this is not the explanation, then it must be something else. But it's saturday evening and my brain is off for the weekend...


regards
Patrick