Mileage is a number that originates in the marketing department. It's useful for sales people who have to relate to customers who understandably have no knowledge of the standards based measurements expressed in decibels.
Here is the manufactures published specifications for the
HD7694. Notice there is no reference to mileage... This information comes from the folks in engineering. I have a high degree of confidence that if I were to use calibrated test equipment, I would see a production example produce theses gain and directivity results.
Other manufactures such as Antennas Direct, Antennacraft, RCA, Wade, Blonder Tongue, etc. offer similar information. With the predicted signal strength tools TVF offers and trustworthy data from the antenna vendor we can calculate with far better accuracy than if we used a vague or arbitrary mileage estimate.
I have very little confidence in antenna products that only offer mileage claims.