Resource Assignment split

I have input some resources which are non-labour. The activity type is fixed duration and units. To start with the activities where Level of Effort, but then i changed them to Task dependant and ammended the dates. (i made sure that resource dates and durations matched the activities)  I then noticed a problem.

When i look at the activites on the resource usage spreadsheet, the costs differ slightly than on the activities screen, normally by  between 0.01p - £7. Not a huge amount admittedly, but im wondering why this is happening. The actvity durations are between 15hr - 27hr and they dont span long periods of time.

Has anyone any experience of this in P5. I am getting around it by removing the resource lines and re-adding them, it then seems to be perfect - but i am wondering what caused it. Any help would be appreciated.