I have an AP driven by a RB711 feeding a single-chain 15dBi omni.
USA FCC rules (assuming I'm not making some stupid mistake in interpreting them) say I should be able to use up to 21dBm on the radio. Reference: http://www.wlanantennas.com/faq.php
If I set the interface to "regulatory domain, United States, antenna gain 15," RouterOS sets the transmit power to 12dBm. Same if I change "United States" to "no country set".
Why does it do this?