Average speed for ride

When I plan a ride in Connect and set my desired average speed or goal time, my 1030 Plus changes the average speed. After it is first uploaded to the unit, the goal time is correct, but as soon as I select the route it updates the goal time / average speed to a faster speed. Whilst I can set the Virtual Partner to a lower percentage to actually get the right speed, why do I have to do this every time? How can I stop my unit updating the average speed?