Two sources, two meanings
GPS adds up the segments between location samples. The vehicle odometer uses vehicle data and rounds the result according to its design. Signal loss, point filtering and manoeuvres may increase the difference.
What goes into the report
The formal register uses the distance derived from odometer readings. The GPS track helps describe the route and provides statistics, but should not independently correct the odometer without the user's knowledge.
Good practice
The app may suggest a final reading, while the driver or administrator confirms the actual value shown in the vehicle.