According to a Wii U white paper the dev kits being used for the Wii U prior to E3 were underpowered.

In their E3 conference Nintendo showcased footage from the PS3 and Xbox 360, explaining that the Wii U would be as powerful, if not more, than them and that developers were still getting used to the console. It resulted in some criticism but according to a new report the real reason was because the development kits being used were underpowered.

Hit Detection, a consulting company, has written a Wii U white paper and suggested that prior to E3 developers were working with underpowered development kits. The report claims that "developers have underclocked development kits, and worked hard to deliver titles running on that hardware to demonstrate live at E3."

According to this report there could have been real demos from third party developers but due to this reason Nintendo chose to use PS3 and Xbox 360 footage. There is no information about how much more powerful current Wii U development kits are although there is plenty of speculation that it could be about 50% more powerful than the PS3.