$250 spent doesn't always equal exactly $250 of shippable ammo. When we calculate our Free Shipping trigger we look at the ammo that we would ship to you and is made up of shippable factory boxes (ie: 50 rounds of pistol or 20 rounds of rifle) because we don't ship loose rounds. You may have $250 worth of ammo in your inventory but only part of it is shippable at any given time.
Example: You have 346.2 rounds of pistol ammunition in your inventory. Only 300 rounds of this is shipable and will be included in the calculation for Free Shipping. The other 46.2 are considered a partial box and won't be included in the calculation until more is added from a future order and it makes up a full 50rnd box.