Why does distance change after you stop recording a hike?

The short answer

The number you watch climb during a hike is built by adding each new point to a running total the moment it arrives, so at any instant it reflects the points received so far — not necessarily the very last one, which may still be a beat behind. The number that lands in your history once you end the hike is calculated from the finished track as a whole, point to point, start to end, with nothing left half-added. The two are close almost all the time, because they are built from the same points; they are not always identical, because one of them was still in progress the last time you looked at it.

How the number on screen is actually built

While a hike is recording, distance is not something that gets fully recalculated every time it changes on screen. Instead, each new position gets compared to the one before it, the gap between the two gets measured, and that gap gets added to a running total — a small amount of new distance tacked onto everything already counted. That is an efficient way to keep a number moving in real time, but it also means whatever you are looking at is only ever as current as the last point that made it into the total. A position captured a second or two ago, still on its way to being processed, is not yet reflected in the figure on screen, even though it will be part of the hike either way.

What actually happens the moment you tap stop

Ending a hike closes off the list of points for good and turns it into a finished, static track rather than a total still being added to live. Whatever the app does with that list next — checking it, saving it, working out the final numbers for your history — happens over the whole thing at once, point to point, rather than one increment at a time. Any position that was captured right up to the moment you stopped is included in that pass even if the on-screen number a second earlier had not caught up to it yet. That is usually the entire source of the gap: not extra points appearing from nowhere, and not points quietly vanishing, just the last stretch of the walk being counted once and for all instead of glanced at mid-update.

Why this is expected rather than a sign something is wrong

A live number and a finished number are built by two different processes for two different purposes — one to give you something to glance at while you keep walking, the other to settle on a figure worth keeping — and expecting them to always match at the exact instant you stop is really expecting a number still in motion to already equal a number that has not been calculated yet. The gap this produces is small, almost always a matter of a few metres or a fraction of a unit, because it comes from the last handful of points rather than from anything being recalculated differently for the whole route. Nothing about the recorded track changes underneath it; the points are the same points, just added up at a slightly different moment than the one you happened to be watching.

Where TrailCam fits in

TrailCam keeps distance, moving time, pace and elevation gain running while a route records, and shows the finished totals once a hike is saved into your history. A route also exports as GPX 1.1, with a position on every recorded point, so the same list the final distance was worked out from is the same list any other app reads if you export it — nothing about that last stretch of the walk is treated differently depending on which number you happened to be looking at. TrailCam is free to download.