sap not following its own rules

i made a vendor recon account and assigned a sort key 010 to it. then i created a vendor master record and in that also i gave sort key as 010. now i made a po then grn then miro.

as a result of simulation in miro i got an entry vendor recon account cr with no po no. in purchasing doc column and gr/ir debit with po no. being displayed.

when i did fbl1n for this vendor account i didnot get anything in assignment field while it was expected that po no should be reflected in assignment field as sort key assigned to this vendor master record was 010

why is this happening SAP is not following its own rule that what ever is given in sort key what we see in assignment field

please explain with some solid proof