The 84mW max. is more a matter of physical/electronically protecting the device.
Devices that were Not engineered for High Signal output tend to Amp. the noise together with the Signal. Device with (example) 50mW output and low SNR will function better than devices with 84mW (or more) if the the SNR of the higher out put is off.
The average End-Users Wireless is designed and works at about 33-40mW, the sweet spot of many of them End-Users is about 60mW.