That was my guess also.
And there have been times when I've been hypermiling my WRX on the highway by doing the speed limit or a mile or two an hour faster than the speed limit in the right lane and people appear to be angered by it. My take on it is they aren't paying for my gas so they can go about their business and don't worry about me. Plus, they pass me and the other sensible drivers thinking they are getting somewhere, yet a few miles down the road I pass them right back as they slow to exit the highway. Moral of the story is that speeding doesn't get you anywhere unless you are going to sustain that speed for quite a while.