Whenever I refer to distance or the miles associated with a trip, I’m actually speaking of “Composite Miles” which WMATA uses to calculate fares.
"The composite mileage is the average of the track miles between any two stations (the miles the train must travel) and the straight line distance (or "as the crow flies" distance). As such, trips between stations that are geographically close but require long travel distances are discounted. For example, Wheaton and White Flint, both on the Red Line, are approximately 3.5 miles apart using straight line miles; but a trip between them on Metrorail is nearly 24 miles. Therefore, the fares between these two stations are based on the composite mileage of approximately 13.75 miles."
Source: A Note on Mileage