Interstellar clouds absorb some of the microwave background radiation.
There are more quasars near the center of the universe, making it hotter there.
The older portions of the universe, which lie in the center of the universe, are slightly cooler than the younger portions.
Earth is moving in a particular direction with respect to the average of the rest of the universe.
More stars cluster on one side of Earth than the other, making it hotter on average there.