@kchris10 

You may want to look at the inventory history for items in the build assemblies.  If any of those items on-hand quantity went negative (were oversold), and were then received on a bill or the inventory was adjusted, QB will make retroactive adjustments to COGS for the oversold items if the cost on the bill or inventory adjustment differs from the average cost used to calculate COGS on the oversold items.  It makes sense because when you oversell an item, QB has to use the last known average cost.  If you then subsequently receive an item at a different cost, an adjustment should be made as of the invoice date or inventory adjustment date to reflect the updated COGS.  I can't say for sure that's what's happening but it's a place to start.  Allowing inventory go negative is something you want to avoid.