It all has to do with liquidity. Supply vs. demand (so to speak), size of bids and offers available on the book vs. size of market orders eating away at them.
During after hours, liquidity is thinner and it takes less effort to move price.
If you'd like a more in depth answer, I've already painstakingly explained the process here:
http://www.forexfactory.com/showthread.php?p=7957450#post7957450
Focus on parts 1, 2, and 3, specifically in the DOM price discovery process of part 2 you will understand the answer.