Hi,
MB5B will show all valuated MM & FI documents which are impacted the material stock valuation.
e.g. MR21, MIRO postings credit & Debit entry to material stock account. But these documents can not be seen in MB51.
Double click in MB5B on material code line & then you can see all MM & FI documents. There you can filter the documents which are not linked to MM movement documents to analyse further.
regards, rakesh