退休金帳戶的法律資詢

Hi, I posted a question about my investment at TIAA-CREF, the original post at:
http://web.wenxuecity.com/BBSView.php?SubID=law&MsgID=60304
I found my account blance had an abnomal drop, from 09/30/2006-12/30/2006. After my constant inquiring, they recongnized that the droped money was caused by speed and diligence from their webteam. And I was reimbursed my loss. I summary a letter from a Senior Individual Consultant about the amount they return to me as below:
According to our records your accumulation totals for specific date are:
9/30/2006 $14,125.83
1/12/2007 $16,382.02
1/18/2007 $16,645.85
2/1/2007 $17,293.10
it seemed a good ending. But I just checked my tax form by TIAA-CREF, it clearely showed my total fair market value as 12/31/2006 was $13843.05. From 12/31/2006-01/12/2007, I calculated the average return of the items I investmented is less 4%, but they returned me $2539, 15% high. If the mistake caused by speed and diligence from their webteam, how to explain the data on 2006 tax form they sent to me? How can I make sure what they did is legal or not? Hope anyone can give me some ideas. Thanks.