quota arrangement

hi

I m very new to quota arrangement.

I cearted new material and maintain info record and source list for the vendor A and B.

and i maintained the quota 40 for A and 60 for B.

now i created one PR for 120 Qty and assign in T-code me57 then system assgin the Vendor A, and when i am creating PO with referance to PR system giving the warning massage ' in quota arrangement the vendor should be B

My question is why in T-Code system assign the vendor A?

Regards

Nitin