Some Miners mine empty blocks, usually if it's found before they have time to fill it up with transactions. Empty blocks are usual mines within a minute or less of the previous block. Because these blocks can't have transactions in them they bring down the full block average, these blocks do not contribute to the capacity of the network but are reflected in the average. Some miners limit there block size to the 750KB dealt recommended by the Core Developers, these blocks are 100% full at 750KB, this also  skews the average.

its worth noting when Gavin first proposed making bigger blocks the biggest objection from the Core developers was its not needed and if it is ever needed the limit could be changed quickly. well it's needed and they now saying next year.