problems on update data date

Hi:

When I update the data date in my P3 schedule, My steps as follows:

1. update all activities weekly

2. update data date weekly

3. schedule

Problems:

The early finish dates (already actual started activities) also prolong one week after I updated the data date. And the early finish dates is later than data date. I think these acutal status activities should not move when I update the data date.  So that I need come back, and carefully check the started activities. And update the changed activities.

Any one knows what problems I have made?

Thanks for your help!