Pur Grp issue in ME11

Dear Guys,

Could you tell me the reason for the above thing.

as per my knowledge, the PR is created with pur group say AAA. Later i save the PR. then i assign the vendor in ME56 tcode. For this vendor the inforecord is created with the price and the Pur Group is given as BBB.

I try to create the PO, then AAA pur grp is coming. So it is copying the PUR Grp from the PR. That is correct.

But my question is under what scenarios, the BBB pur group will come into Picture.

Explain me this issue