Are you able to drill into the reports that you are using so that you can see the information that it's looked at?
It sounds like you could be right and that it has counted some of the records multiple times due to the time changing. I would suggest logging this with your Support Provider who will be able to run some tests on this and see if this is something that can be replicated and logged as a defect.
Thanks Jenny. I may be able to dig into it later. We're fighting a number of other issues at the moment so I'll have to add it to the stack Fortunately this particular one went largely unnoticed here.