If your track can accommodate 20 Amps on each circuit, why do you advise only using 80% of the capacity?

Each circuit can fully handle 20 Amps, but because of field installation conditions such as the quality of existing building wiring, track distance run, number of connections, etc., we advise de-rating the track by 20%.  An additional benefit to using a conservative load factor is that you have the opportunity to add new fixtures to your track in the future if needed.