Manufacturing Module: Items unexpectedly added to MRP exclusion list

Hello,

We use the manufacturing module and we've found that items are frequently showing up on the MRP Exclusion List automatically. These are NOT kits, services, miscellaneous, or flat fee type items that can't be removed.  We never add any parts the exclusion list, yet items that we purchase regularly through MRP end up on the list. 

Any thoughts on how this may be happening? Is there a setting elsewhere that will put an item on this list?

Thanks