Adjustments export: Why are the adjustment_product_code and subscription_ID fields empty?

If you're looking through the Adjustments export, you may have noticed that some adjustments have empty adjustment_product_code and subscription_ID. Why?

The reason why the adjustment_product_code fields are blank for some adjustments is because these are line items not associated with a subscription charge. This is also the reason why those same adjustments have blank subscription_id fields. If you look at column U (adjustment_origin)​you will notice that the adjustments are either:

1) debit = refund
2) carryforward = carryforward credit (credit that was higher than the invoice total was applied and the left over amount sent back to the account) 
3) credit = credit adjustment that was invoiced and used up
4) one-time = a one-time charge not associated with a subscription

Only adjustments that are subscription charges would show those two fields populated.