System Quantity in the Count Adjustment Approval Summary form does not match Quantity On Hand

We are having an issue when approving Count Adjustments where the System Quantity on the approval form does not match the Quantity On Hand .

For example we have an item where the QOH = 150. When we do the cycle count we have 450. Then the Approve Count form will show the System Qty = 0 and the Adjustment Qty = 450. Once the adjustment is approved the QOH = 600?
This is not the case for all items but there are a lot of them that are like this. How does the form calculate the System Quantity and why would it not match the Quantity On Hand?