By default, the device trip computer collects position data once every second.
It then adds up the total distance travelled based on those one second increments, and that is the distance shown on the trip computer’s data field for distance walked.
The track log though, (when set to Record Method auto), only records data once every several seconds, so unless you’ve walked in a perfectly straight line between each two consecutively recorded track-log points, the log will very slightly, but continually, be under-recording the distance travelled. - It’s effectively recording the straight line distance between each consecutive pair of track log points, whereas in reality the walker wouldn’t have walked in a perfectly straight line between the logged points.
When you upload the data from the GPS device to Basecamp, the properties summary shows the total distance as the figure that was displayed on the trip computers data field, based on logged implements of 1 second apart.
However, the GPX track-log, and associated graph, show the data that was recorded by the track-log, based on individual data points, each being several seconds apart from its neighbour. Thus accounting for the reduced total Length showing on the Graph and the recorded GPX track.
Video instructions and top tips for everything you need to know about Garmin BaseCamp can be found in the GPS Training online resource - more here