That is an amazing difference. Here is what someone sent me when i asked a similar question:
"I'm sorry to hear about the issue with your refund amount. It is hard to determine what happened since you haven't received a letter from the IRS or BFS.
It is possible that apart of your refund was offset by The Department of Treasury's Bureau of the Fiscal Service. If this was the case, you will receive a letter in the mail with more information. You can always contact BFS regarding this as well.
I've attached a link below with more information, as well as contact information."