The gauge on your display that shows the battery level can vary in the short term based on how much current is being drawn by the system.
What you are experiencing is most often attributed to "voltage sag"-- a temporary reduction of the voltage in the battery. The battery gauge on your display panel is actually a simplified voltmeter and will reflect this dip in voltage. Voltage Sag is most likely to occur during increased loads-- like a hill climb, and/or when in "electric-only" mode (throttle only). After the load is returned to a lower level (for instance-- when you reach the top of the hill), the voltage/gauge on the display will rebound fairly quickly.
You will get a better indication of how much charge is left after that rebound has occurred.