How is it that after staying behind someone on the Interstate who is driving 20 miles per hour under the speed limit (giving them plenty of time to adjust to the traffic flow, or so I thought), when I try to pass I can barely get even with them even though I’ve accelerated more than the 20 mph they weren’t going when I was behind them?
It’s a mystery.
It's a long story ... 
maybe it’s based on the theory of vehicular relativity. The closer you get to passing a moving vehicle, the faster it moves so as not to be passed.
Oh, so that explains it!