Having known several people who do that, I've come to the conclusion that yes, it is a genuine attempt to keep a certain speed, but NO, they don't realize they're doing it. It seems that they just have numb feet (and numb brains). They accelerate from a stop, notice that they are going too fast, and take their foot off the throttle. The car slows down, so they give it more gas. It speeds up, so they take their foot off... it makes the passengers want to hurl, and it murders your fuel economy as well.
I live on a slight hill near a fairly busy suburban road. One car in particular I notice almost every day - WAAAAAAHHHHH out of the intersection wuuhhhhh through the slight bend WAAAAAAHHHHH at the base of the rise wuuhhhhh halfway up WAAAAAAHHHHH over the top wuuhhhhh down the other side... if I rode with that person every day, I'd kill him.