Hombre, he was probably pacing you and using the radar to give him his speed. Most units I am familiar with can't do same direction while moving. Otherwise they have to be stopped or if moving, on-coming vehicles only can be measured. So if you would have punched it and pulled away he would have to estimate your speed. Try it next time see if he gets it right.
OK time for a Flip-type of tale...... Radar detectors are legal in Missouri. One time I was driving a very slick unmarked Crown Vic, it had regular license plates and the lights were LEDs mounted behind tinted windows. I had to make a quick trip to our state capital, about 150 miles away, for the agency I was working for, so I drove it. On the return trip, this gal passed me. I saw she had a radar detector, so I flipped on the radar unit in the car I was driving, she immediately slowed way down which caused me to pass her and get ahead of her a ways (it was a four lane road). About 10 mins later here she comes again, this time I let her get further past me before I hit it, and I left it on for a good 30 secs. Of course she slowed way down again, and I passed her again. This went on about a dozen more times over the next 80 miles before she finally figured it out.