Well, this comes down to the expansion mucking things up with regard to distances. The article Spourk linked to is probably a good explanation, but I'll offer a shorter one here.
The expansion of the universe increases the distances between objects in the universe, no matter what those objects are. This even includes light rays. That is, if a light ray is emitted some 500 million light years away, then the expansion of the universe will, each second, cause more distance to be added between us and the light ray. So that in the end, over time, that light ray has to travel more than 500 million light years in order to finally reach us. How much more? Well, that depends upon how fast the universe is expanding, and that changes over time.
For example, when the CMB was emitted, our universe was expanding so fast that stuff that was, at that time, some 43 million light years away has actually had to travel a full 13.7 billion light years in order to get to us. So we're seeing bits of the CMB today that were at that time only 43 million light years away. But the universe has expanded since then, by a factor of 1090, so that the stuff that emitted that light is now a whopping 46.6 billion light years away.
This may seem counter-intuitive, but remember, the expansion has carried the light ray away from us fast enough for it to need to cross 13.7 billion light years, but that light ray has been moving towards us the entire time. The matter that light came from hasn't! So the same amount of expansion has been applied to a larger distance between us and that galaxy than us and the incoming light ray, so that it was pushed away even further.