In my opinion, the Inventry Adjustment account is used to post inventory quantity/value adjustment transactions. However, a bunch of my invoices post records to this account. In addition to this, I found that if I make inventory adjustment, it will affect the amount of the "inventory adjustment" on those invoices in the past. I don't understand the logic behind all of this. Can someone explain to me, please?

Here is an example of those invoices. I cannot see why there is a $20 inventory adjustment here. If I adjust inventory quantities of the items on this invoice, this $20 number will change, and the amount of change seems to be very random. I cannot understand at all. Help me please and thanks.