FCC Requirement for device to device output power variability?
As far I know, FCC only limits the maximal EIRP power (mean value in all directions, measured @3meters). So no problem to be +/-1dBm under condition that the most powerfull device stays under limit (And it should not be a problem if you follow the reference design).

This Nordic White Paper gives some information about regulation.

However to have precise information about the requirements you should contact Nordic or you local certification lab. They are most used to this.