Originally Posted by
channa
It's even more complicated than that. Say there's a 20-minute crew issue, followed by a 4.5 hour mechanical, then a 10 minute delay because the water had to be topped off after that long mechanical.
A "reasonable person" would say that was a mechanical delay, but UA might display it as preparation because that was the last thing that happened to it.
Taking nothing away from your point, my experience is that it's usually the first 'thing that happened to it' that sticks. E.g., delayed due to late inbound aircraft, then returned to gate to disgorge a passenger, and they will only report the inbound aircraft.