All the speed and distance calculations in RGT are based on your power. There are a few variables that contribute to the algorithm, such as your weight, the current slope of the road, drafting benefit, air resistance, rolling resistance of the road, etc.

On a bike computer the distance is calculated based on your wheel size and wheel rotation speed. Slope, weight, resistance and other variables are not taken into account, leading to discrepancies between RGT and your bike computer.