There are certain use cases where you set "User A" to the Developer Roles on Object Type A - this will allow the users to edit ALL the Child OTs, but if the admin (or schema manager) sets another User (User B) as Developer of Object Type B - this will have priority over the Object Type A Roles.
It means User A will NOT be able to edit the Object Type B objects.
the "Object Type B" objects will show the Edit button - but User A will not be able to save or edit, due to the "lack" of permissions caused by the above situation.