I'm not here to state if the Wii-U is more powerful or not, because I can't provide proof for you, but I'm sure that these games are using ported engines, engines not built up from scratch with the Wii-U in mind.
The other consoles, where the majority of the games and engines will come at the beginning are all GPU bond hardware, while the Wii-U is a CPU bond/limited console, so one would need an entirely different approach and solutions to get the best out of it. (It would be nice to see Rage on the Wii-U for example, I wonder if it would be possible to use GPU-transcode to "help" the CPU, like how it's possible on the PC)
I'm sure that sooner or later devs will learn how to achieve the same or better visuals what you can see on the xbox360 or on the PS3 because the GPU is probably more powerful (I assume).