Why do two phones show different tracks for the same hike?

The short answer

A phone's GPS receiver never compares notes with anyone else's. It works out its own position alone, from whichever satellites it can see at that instant, through whatever sits between it and the sky at that instant — a jacket pocket, a hand, the leaves overhead. Two phones walking together see almost the same sky, but not exactly the same sky, and a GPS fix is sensitive enough that 'almost' is enough to draw two tracks that are close but not identical. A gap of a few metres between two tracks recorded side by side is ordinary and expected, not a sign that something went wrong.

Each phone is solving its own problem, alone

GPS is a one-way broadcast. Satellites send a signal down; a phone listens and works out where it must be standing for that signal to have taken exactly as long as it did to arrive from exactly those satellites. Nothing in that process involves talking to another receiver, sharing a result, or checking what a nearby phone came up with — there is no channel for that, and none is needed for the system to work at all. Two people walking the same trail are simply running that same calculation twice, on two separate devices, and each run is free to land on a very slightly different answer.

Why 'almost the same sky' still isn't the same sky

A signal from a satellite can arrive fractionally differently at two points a metre apart, because it may reflect off a rock face, a tree trunk or a wall differently for each one — an effect that grows the more of the sky is blocked to begin with. Two phones a stride apart on an open ridge will usually agree closely, since there's little nearby for the signal to bounce off. The same two phones under tree cover, in a narrow canyon, or close to a cliff face can end up disagreeing by more, because each one is picking up a slightly different mix of direct and reflected signal.

Where the phone is carried adds another difference

A body blocks part of the sky on its own, and how much depends on where the phone sits — held in a hand, sitting in a chest pocket, clipped to a backpack strap or buried at the bottom of a bag all give the antenna a different view upward. Two hikers on the same trail rarely carry their phones the same way, so on top of the small differences GPS reception always has, they're also handing their receivers two different partial views of the sky to work from.

The distance totals will differ too, and that's expected

Total distance is added up from a track's own recorded points, one short step at a time, so it inherits every one of the small position differences above. Over a short walk that adds up to almost nothing; over a longer hike, a difference in the low single digits, as a percentage, between two honestly recorded tracks of the same walk is ordinary — not a sign that one phone measured correctly and the other didn't.

When it's worth a second look

A small, fairly steady gap that holds for the whole walk is the normal picture. What is worth checking is a track that agrees closely for most of a hike and then jumps well away from the other one over one specific stretch — that points at something local to that stretch, like a section under heavy tree cover, a run along a cliff base, or a narrow street between tall buildings, rather than at a difference between the phones themselves.

Where TrailCam fits in

TrailCam records a route from whatever GPS signal reaches the phone at that moment, the same way any GPS recorder does, and exports the result as a GPX 1.1 file or CSV as part of the optional TrailCam Pro upgrade — the app itself is free to download. Comparing a hike recorded in TrailCam against a friend's recording on a different phone or app is reading two honest, independent measurements of the same walk, not checking one against a single fixed correct answer.