Material Costing: Moving Average Price Issue

Hi Experts,

I have a issue regarding the Moving Average Price of the Material computed by the system (indent sales process):

The Material was created with Purchased Price .74PO Price x 10 items = 7.4 Amt in LC Posted into the system (PGR).

But afterwards, they reversed the Material Document with the same Price .74PO Price x 10 items.

This had been done last 2009.

Come 2011, they created another indent sale -> PO ->PGR for this material, now the price is .12 PO Price x 10 items = 1.2 Amt in LC Posted. However, the moving average price was computed 0.77

How does the system comes up with this computation? Is there inconsistencies in the system?