Strange values in Fixed Cost timebased data?

[JimS]

I've found the following situation with my plan.

I have a task that only has a fixed cost on it (no resources). If I go to
the Task Usage sheet and look at the timebased data, I can add up the
timebased cost entries for the task and take away the sum of the timebased
actual cost (to get a 'timebased' remaining cost value). If I have updated
the task to 50% with a button then the 'timebased' remaining cost value is
the same as the task remaining cost value. But if I use the 'Update as
Scheduled' button then I can find a large difference between the values.

Has anyone experienced this and are there are reasons?

Cheers
JimS