Why is the prediction for an official course dead-on for me, but too pessimistic for my own course?

The elevation gain/loss of a course is an important factor when predicting your finish time. The official, preloaded courses for the Race Power Calculator have extremely accurate records of the elevation gain/loss for that course.

When you upload your own course, the quality of the data is important. If the elevation value from the FIT file you upload is noisy, this leads to an exaggerated elevation gain value. This makes the Race Power Calculator think that the course is more difficult than it is in reality and leads to an under prediction of your finishing time. The target power value will still be very accurate, as your power target for your race is the same regardless of elevation change.

If you would like to upload a higher quality elevation trace of your race course, you should find an activity sourced from a watch that contains a barometer and GPS. Typically, watches that contain a combination of a barometer and GPS produce an elevation trace that is more representative of the actual course.