Project Roles and access

Hello, We are in 11i. We have 3 different roles in project: Project manager, Project Buyer and Project Accountant. They are configured in the same way- That is: the same control actions in Role setup for the 3 roles. They access to the same responsability. But they can not do the same. For example Project Buyer can't change project status. I don't see why not since they are accesing the same responsability and even when they have different roles, they are defined  exactly the same, Thanks.