Moving average's "Price difference for moving average" in AX 2012

The moving average is brought in AX 2012 feature pack which is commonly used by retailer. As 'moving average' is a perpetual costing method, user doesn't need to recalculate the invent cost of happened transactions, because the cost is automatically averaged by posting purchase order receipt/invoice. Then the question comes, how does AX handle the price differece between receipt and invoice when there's sales invoice posted between these 2 trans?
The answer is that the difference will be posted to account "Price difference for moving average".
I created an item 'test1201' which uses 'moving average' costing method and configed the posting profile,then I did
1. Create a purchase order of 'test1201' and post receipt at price $100.
2. Create a sales order of 'test1201' and post packing slip and invoice.This is the vouchers of invoice,you can see the inventory cost is $100.

3. Post invoice of the purchase order at price $200.This is the vouchers of invoice, and you can see the voucher of "Price difference for moving average".
As you can see the difference is posted to "Price difference for moving average", and the invent cost of sales order is not updated at all. The amount in "Price difference for moving average" will accumulate everyday, or we can do something to manually revaluate the inventory cost of that item and settle the amount in "Price difference for moving average". Will talk about that later.