Your Hangar 9 meter is probably putting a load on the battery, which simulates the receiver and some servos. This lowers the voltage. The multi-meter does not put a load on the battery, hence the higher reading. To confirm this, and the accuracy of your Hangar 9 meter, hook up the hangar 9 meter. Then use the probes on the multi-meter to check the voltage, perhaps where the banana plugs connect to the Hangar 9.