Fleet Genius reports the data collected from your VHMs or the data you manually enter.
The VHM calculates your mileage by integrating your vehicle's speed every second. From that information we can calculate how far your vehicle has moved over the duration of a trip. Your vehicle's manufacturer has calibrated the speedometer at the factory based on a set of standard specifications include wheel/tire size, inflation, tread depth to ensure that your speed is accurate.
Not all vehicles odometer readings are calculated using speed - some have more mechanical measures. In cases where odometer is calculated from methods different than the VHM there can be what appears to be odometer drift.