Difference between MB51 and MB5B

Dear,

When I run report MB51(Material document for material)  for a material/plant and posting date system shows me 6 document posted however when I run report MB5B (Stock on posting date) for same material/plant and selection date for a year it shows me only 2 documents, can anyone know the differences between these 2 reports if any.

Regards,

Manmohan