The best that I can think of is who is concerned with the information you are giving. When a GA aircraft gives ATA and ATD that allows others to see how long the aircraft is in flight. But on a commercial carrier the passengers don't care if they landed on time, they want to make sure they get off the plane when they are suppose to, so to allow for easier planning for customers airlines factor in taxi time for them. This isn't supported by any information, its just the most logical explanation I could think of.