The Trackpoints are red on the track of a walk shown below
Active Memory (left) and Stored Track (right)
Only the minimum number of points needed are stored.
Look at the canal towpath in the bottom of the images.
Only 2 points define it; replacing over 20.
The Active Memory contains all the points that have been recorded. When the Track is saved an "intelligent" routine eliminates any points which are not essential to define the shape of the track.
In the example here the track was recorded on a Garmin eTrex 10 and the "intelligent" routine was an embedded party of Tracklogs digital mapping software. The eTrex 10 does not have any built-in compaction routines.)
The apparent length of the track will be very slightly reduced in the storage process as the measured distance is now "cutting the corner" slightly.
