resource spreading

Alright guys,

I split my resources (GBP) in my program into periods that i have set up in the financial period data. I have ended the year on 31/03/2012. I have got activities with costs assigned with the finish date on the 3rd March ( planned finish date is 5th March), however when i look at the resource assignment spreadsheet it is carrying over a proportion of the monies into next year. Does anyone know why this is happening? I know how to reset it - i delete the resource and recreate it. Im just wondering how it happens as the finish date is nowhere near the end of the yeat.