The most likely explanation is that your lender pulled the TU04 version of FICO, but the score that you get from this website is the TU98 version. The formulas are slight different, and many people have reported wide differences in score between the two. In some cases, TU04 is higher; in other cases, it's the other way around.
So, your score probably didn't really drop. Mortgage inquiries are excluded from FICO scoring for 30 days, so that when someone goes to a second or third lender to shop for mortgages, those lenders don't see lower scores because of the initial inquiry.
