Schedule Adjustments

I am entering %complete using the Update Task tool. I'm entering actual and remaining duration.
However the resultant date that is then calculated is wrong.

E.g task of 20 days that was due to start on 06/07 and finish on 02/08. It actually started on 08/07, and is as of the status date at 15/07 only 5% complete. I have entered actual start 08/07, 1 actual duration and 19 remaining. This should push the date out to 11/08 (19 days from status date 15/07, but MSP calculates this as 31/08.

It miscalculates on a number of tasks where progress is either ahead or behinf schedule or has not moved since the last status date. However on some other tasks, it calculates perfectly correctly!

Please help!