Almost certainly the mileage entered into Motability OLA (Online Application) at the hand back stage of the lease. Or from the MOT.
They need the final mileage for when they sell it to BCA or on their dealer direct website to calculate what reserve / sale price the car needs to be offered at.
Then it’s just a case of dividing the final mileage by the number of days the lease was in-life (so for a perfect 3 year lease with no overage – 1092 days; aka 39 weeks) to get the daily. Add up every daily on the scheme and divide by the number of cars.