Hmmm. In that chart, the average for non-hybrid was within 1 MPG of the low end MPG (with only one exception of less than 2). For the Hybrid the difference was 10MPG (20 for Highway).
My understanding is that for the hybrids, the battery acts as a test loophole, since it is fully charged at the beginning of the test and somewhere between the recharge point and full at the end of the test - thus acting like a little extra gasoline in the tank. The highway test (which starts with the car having already been running) seems to reflect both this, and the relative lack of efficiency at speeds above 40 mph or so.
That's funny-- yesterday I drove my Prius on a 244 mile trip, mostly highways and interstate-- I got 57.1 mpg average-- with my bags and two passengers in the car with me.