If you look at the map, the GPS thinks you are out of lane 1 (looks like lane 12 at some points), so that's where the extra distance comes from.
You can trigger laps manually in workout mode, and while I haven't tried this, maybe the thing to do is to change your interval distance to something longer like 0.5mi, always hit lap after the interval, and then it will move on to the rest period. The start of your next interval should still trigger automatically after 2 mins rest.
The track is hard for GPS, but check out one of Jagge's sessions where he's using a head-mounted antenna:http://www.attackpoint.org/sessiondata.jsp?session...
(not that I am suggesting this)
Alright, next time I'll wear it on my hat.
When I went to the track Tuesday, all six 400's were spot on at .25 miles with the watch on my wrist: the gps track never leaves the inside lane. I think it's just a matter of being locked into enough satellites.