I don't understand why the "Available" column on an existing sales order shows an amount that is equal to the On Hand quantity + the line item Quantity. 

For example, when entering a new sales order, the item has:
On Hand qty = 1306
Available qty = 1306

This I understand, because the item has not been committed on other orders such that the Available quantity would be different than the On Hand quantity.

But on existing (fulfilled and invoiced) sales orders, it shows:
Line item qty = 50
On Hand = 1306
Available = 1356

I just don't get this.  Can somebody enlighten me?

Thanks,
Cecelia