An organiser's Track can often be a recorded tracklog, which may itself go for a wander every so often, instead of staying close to the road. It's not a good way to produce a Track intended for distribution (drawing it afresh in a Planner is much better) but it is very common.
My Edge 800 occasionally alerts me to being off course like that, despite riding along a fairly straight section of road without tree cover. A few seconds later it'll announce "course found". Not sure why either.
As a cyclist (and speaking for myself as a fairly bulky one, who likes to ride very forward, looming over the bars) you have to bear in mind that almost half the sky is unavailable to the GPS right from the get-go, blocked by your body. The signals are after all incredibly weak - of the order of 50W transmitters, and over 20,000km away.
Now the GPS spec is, I think, that given a completely clear hemisphere of sky view, the absolute minimum number of sats above the horizon (at our sort of latitudes) is 6. A Garmin will lose itself when the number it sees drops to (I think) 3 (for a few consecutive seconds). Even though in theory, once a position has been established using 4+ sats, 3 or even 2 should be enough to maintain a useful approximation.
So even with a clear sky view (which itself is fairly unusual), a GPS with a large human in close proximity can be on the limit.
However, that minimum of 6 is quite an uncommon and short-duration event and most of the time it's 8 or 9, and can go up to 12. That's with the 'set' of 24 satellites. Actually there are a few spares knocking around up there as well, and some of them are turned on, so there's actually over 30 to go at. That's not counting the GLONASS sats, which the latest Garmins can also access, or Galileo if it ever gets off the ground and in the right orbit (the last 2 to go up didn't!)