Vehicles - Discrepancy in odometers

Why are the mileages shown in MyGeotab and on my vehicle's dashboard different? 
 
It is common for odometers to have an error of ±5%.

Odometer errors are usually proportional to speedometer errors.

Speedometers can have built-in tolerances of up to ±10%, with many manufacturers deliberately calibrating speedometers to show high readings to account for possible discrepancies and protect their liability for speeding.

Odometer discrepancies can begin to appear if tires are out of spec, worn or underinflated.
Sources of error due to tire diameter variations include: wear, temperature, pressure, vehicle load and tire size rating.

See how to make a correction to the odometer by clicking here.