In the standard SAP if you receive a goods against external purchase order then based on the movement indicator (B) system picks the WM movement type (101) and GR is done in the storage type 902 defined in the movement type
Similarly if the receipt is against the production order then movement type indicator will be (F) in that case and system picks the movement type (103) in WM and perform the GR in 901 storage type
while in IM both are assigned to Movement type 101 in WM based on the movement indicator it is segregated in movement type 101 and 103 and so the storage type.
Please let me know if the above information is any help