Coverage maps are a best guess estimate of repeater coverage. The reason it is a best guess is that no two ham stations or repeaters are alike. Also are you using a base station, mobile or handheld.
Receiver sensitivity varies, antenna gain and height varies, location varies for mobiles and handhelds as well as transmitter power and loss figures for length and type of coax used.
Add to that the power output of the repeater, height and gain of the repeater antenna, hardline and duplexer loss and receiver sensitivity you can see how difficult it is to produce a coverage map that is accurate for everyone.
However, taking all these factors into consideration and using averages a reasonably accurate map can be produced.