You entered the invoice through the A/P invoice data entry instead of the P/O receipt of invoice entry.

But the PO # is attached to the invoice and it also shows on the PO the amount invoiced. If I had posted through A/P, how would that have happened? 

If you run the purchases clearing report, does it show the quantity (not dollar amount) invoiced?  If no it would appear that bluetopaz is correct.

If you are seeing the quantity invoiced on the purchases clearing report, then try this.

Go to purchase order entry and pull up the po in question.  Increase the quantity ordered by one and save.  Reopen the purchase order and set the quantity ordered back and save.

I'm sure you know that the quantity ordered and quantity invoiced for a drop ship must match for it to go to completed status.