I would go with Ms. Leslie, assuring that the vendor item Lookup window is working as expected. Let me shed a light on few concepts related to this window,
When setting up an item in Dynamics GP, it can be linked to vendor and given specific vendor item names. On the shipment invoice transaction, choosing a specific vendor will bring only his items either to the item lookup, or vendor item lookup.
These details can be found IV00103 | Item Vendor Master, I believe the vendor has been linked to several items which is why it brings all the items when you go to the lookup either on the shipment invoice or detailed item window.
Your feedback is highly appreciated,