WO Completion generated disturbingly large variance - why?

We had a WO for a quantity of 41,100 pieces of an item with a costs of approx $10. 

Multiple completions were performed with the final completion of 632 pieces being performed on Oct 8. 

We run manufacturing accounting every night - P31802 & P31804. 

The P31804 report for that date show that it generated a variance journal entry for $22,603.32.

What do we need to look at to determine why such a large variance was created?

Thanks

Bob