No way to say for sure without looking at both tax returns as filed.
$400 came from the Making Work Pay Credit, however that was reflected in reduced withholding for most taxpayers.
The short answer is that you over-withheld by $1,600 in 2010 and only $556 in 2011. But without all of the numbers in front of me, I can't be more specific.
Edit: Again, without seeing ALL of the numbers, there's no way to say.