Hi David,
It is very simple logic, until unless the below profit centers are not created for company code YY01 it will not allow you to post the profit centers for company code YY01.
XX01AAAAA
XX01AAAAB
Same logic applicable for company code XX01 also.
It means that, Four Profit centers has been created for both the company codes (XX01&YY01)
I think via Substitution rules it is posting other Profit centers.
Thanks