I recall reading about hyper-expansion at the beginning of the bang... expansion faster than light... this could be part of the reason...
Edit:
OK...the thing about expanding faster than the speed of light bothered me. I found an explanation at this site:
http://www.astronomybuff.com/how-can-we-see-galaxies-47-billion-light-years-away-when-the-universe-is-only-13-billion-years-old/From the site:
A somewhat simpler way to think of the expansion rate of the universe is that it expands at roughly the age of the universe to the 2/3 power: AgeOfUniverse^(2/3). Unfortunately, it’s not simply a plug and chug formula, since the expansion is occurring continuously, you need to apply some calculus.
The expansion formula (see linked website page) just takes the ratio of elapsed expansion time to the age of the universe raises it to the 2/3 power and does this over the entire time the expansion is occurring.
What all of this means is that whenever you discuss the size of the universe, you need to apply a scale factor that is relevant TO THE TIME you are interested in. The issue of when is very important because the size of the universe, and the rate at which it was expanding has changed since the universe began.
So, for RIGHT NOW, the size of the universe has expanded to roughly 46.5 billion light years since the Big Bang.
Let’s break down the above integral into some smaller intervals and watch what happens. Let’s use 13 billion years as the age of the universe and let the universe expand for an average of five billion years at three different points in time: 2, 7, and 12 billion years after the Big Bang:
At age of universe = 2 billion light years: the universe has expanded by a factor of (13/2)^2/3 = 3.48
At age of universe = 7 billion light years: the universe has expanded by a factor of (13/7)^2/3 = 1.51
At age of universe = 12 billion light years: the universe has expanded by a factor of (13/12)^2/3 = 1.08
So combining these scale factors over the two intervals above, the universe has expanded to a size of:
(average distance light travels over interval of interest) * (sum of all scale factors).
Plugging in the numbers (we used an elapsed time interval of 5 billion years):
(5 billion light years) * (3.48+1.51+1.08) = 30.37 billion light years.
The 5 billion light year number above is the average distance light traveled in 5 billion years so the units are in light years.
Now, this is a discreet example, taking only three points in time, but already we have a number bigger than 13 billion light years. Since the universe is expanding continuously, we actually need to do the integral above and when you do that, the answer approaches 47 or so billion light years.