Re-importing periods with incorrect data

I recently found out one of my databases has been updating wrong. Loooong before I took over the database. I found a discrepancy between the WBS and OBS numbers. Of course, WBS and OBS should always be the same. What's funny is that all the periods back to July 2010 were messed up. My team and I tried to delete/re-import these periods in order, i.e. Jul 10, Aug 10, Sep 10 etc. The data was still incomplete. So then we tried it backwards. We started with Mar 11 and deleted/re-imported. Without a recalc the numbers were good. We then did Feb 11, Jan 11 and the numbers came out great. After all periods were done we did a full contract recalc (took all night) and the numbers matched across the board.