For instance, you may talk of infinity of all three dimensions (negative or positive) of three-dimensional space. The "infinity" of a three-dimensional space is the space itself.
If being encompasses the space itself, and has power to do anything within that space, it is omnipotent in that space, but its omnipotence may perhaps not extend to - for instance - another plane (which is an entirely different space). Therefore the being which is omnipotent in its own space, may not be omnipotent with regard to some larger space, a space the smaller space may or may not be a subspace of.
So what is then the meaning of an infinitely powerful being which is not omnipotent? I have no idea, really, but it is an interesting concept. Here are some ideas:
The infinitely powerful being may for instance not sense the entire space, but where his senses reach, everything is possible. 
Another posibility is that the being is omnipotent in its own space, but not in regard to some other space.