Erik wrote:

On the freeway I drive 25 meters behind a car of another make, which is clearly running on cruise control. I set my cruise control. We drive at 100 km/h. 10 km later the distance to the car in front seems the same. If we say it has (without me noting it) changed 1 m the variation is still less than 1/10.000.

I think the answer is quite simple:
Erik assumes, that the car in front of him is running on cruise control. That's probably wrong. Why not assuming the following:

Erik drives on the road, approaches a car in front of him, which's driver decided that 100km/h is an adequate speed for the journey. Now, Erik adjust his speed to same speed as his front man. At this moment something psychological happens: The front man sees the heavy pickup of Erik behind him and now keeps his speed synchronous to Erik's. Why?
If the front man is driving faster, then the distance to Erik's car increases and gives him via driving mirror the feeling to drive faster than he actually wants. If the front man on the other hand is driving slowlier, then the distance to Erik's car decreases and gives him via driving mirror the feeling to drive slowlier than he actually wants.

Two mechanisms keep this feedback alive:

1. Erik's pickup is big and gives an impressive signal in the driving mirror.
2. The speed was originally choosen by the front man and he decided that this is a suited speed for the journey. So, Erik's cruise control helped the front man to keep his speed constantly at that speed, he found to be well suited for the journey.

I must confess, that I do sometimes behave like Erik's front man. If I find a car driving constantly at a suited speed, I do either follow him at constant distance, or I will drive in front of him, also at constant distance. This is not planned, of course, but happens subconsciously.

Kai