I actually found that my Garmin 810 was a couple of seconds faster over most trails than my phone. Two friends bought identical Garmin watches, and they were over 15 min difference in an hours ride.My mate rode with his watch and his phone recording the same ride over the weekend, some segments were faster by 30 seconds or more on the phone. Very few were faster on the watch, and only by a couple of seconds. I think it's safe to say that if you aren't running a military grade GPS you can't trust every segment and phones appear to give you faster times in general. So really, the only valid segment challenge is yourself using the same device each time.
I really hope they consolidate segments with our money, it's so stupid when there are 12 of the same segment.