It says 7.4v on the setting because the is the nominal voltage of a 2s lipo and what most use to indicate that voltage range. Yes when charged they are 8.4V, but electronics can only provide what the battery voltage is minus a little voltage drop in the regulators. So a full charged battery is 8.4v and the BEC output is about 7.8V. When the battery is partially used and voltage is only say 7.6V then the BEC output is about 7.2V. It is one of the reasons we personally are not fans of high voltage BEC setting in our setups because it changes as the battery is used, and just makes the servos too darn fast anyway for good steering.