how an Inventory Item gets entered as Non-Inventory Item while creating Purchase Orders

we are using GP2010 and here is the situation which is leading nowhere

• an Item is listed as Sales Inventory in Item Master
• when a Buyer enters a PO and I look at the "Purchase Item Detail Entry" screen the Item Type is listed as " Non-Invnetory"
• When I enter PO fo the same item, it is fine and listed as Inventory item.
• Receiving receives that item as non-inventy but when we do the Invoice matching, it gives an error being an Inventory item it can't be matched as non-invnetory.

Questions

1. Does this have to do soemthing with GL account attached to that item?
2. From where this filed " Item Type" comes from ?
3. is this from Item Master where Misc. gets translated as non-Invnetory ?
4. I see this column in POP10100 table but nowhere else.
5. Is there a way to manully adjust this field ?

Please help

Regards,