Your question is quite understandable, and quite common. Mileage estimates are the product of the marketing department and based on who knows what generalization and assumptions.
TV Fool estimates the strength of each available signal using the known values of the transmitting antenna performance, transmitter power, antenna heights and terrain between the transmitter and receiving location. The result is far more specific to your conditions than any ad copy could hope to be.
Reputable vendors provide performance data from their engineering department for their antennas that allow us to calculate the final receiving system performance.
http://www.tvfool.com/index.php?opti...57#how_to_read
http://forum.tvfool.com/showthread.php?t=109