Like I said the Prius and especially the V simply cannot be driven as aggressively. They don't have the power so it is impossible to deviate as much from the EPA test. Think of it like a normal bell curve. If you have a less powerful vehicle you cut off more outliers that would otherwise drive significantly faster. They simply cannot accelerate as quickly even if they floor it. So the results are skewed which moves the average.
Now companies could tune their vehicles to do better under higher load (maybe your GTI was like that since you did quite well you said), but they can also tune them for hitting the EPA test targets and I assume they do (it could even be argued this is a good idea since the EPA tests are supposed to reflect real life).
I have seem some efficiency maps from ford in the past and they did have significant variation depending on load (I am sure all companies do, but I haven't looked at all companies). If you google "engine efficiency map load" or something similar you can find such test results.