Reasons for difference between Actual costing Run and AVR

Hi

I need small clarfication on following scenario:

I have done actual costing run Jan, feb and march.

Now in march month i am doing Alternative valution run along actual costing run.

But i found small differences in price difference and ERD difference between Acutal costing run and Alternative valution Run(3 months).

Please let me know reasons for gettting the differencess between Acutual costing run and Alternative valuation Run.

Thanks

Appu