Set of books and ledgers

In our 11i instance we had two sets of books (pfi_mxn and pfi_mxn_usgaap) that were created.  When this instance was upgraded to R12, the pfi_mxn became primary ledger and pfi_mxn_usgaap became secondary when looking at it from accounting setup.

Question is how did this happen?  When looking at the setup of those two books in 11i, there's nothing there to indicate that one's primary set of books and the other is secondary set of books.

Please reply as early as possible.

Thanks