Over the course of 300 miles the car could have an average speed of 20mph. Nowadays we spend a lot of time stationary or in slow traffic (unfortunately!). A new car will often sit stationary for a while while we work out how to turn something on / off 😅
If you take it on a decent drive it should start to increase.
There should be a way of resetting the average speed, but don't ask me 😃