Because the stock EAS calibration numbers in each vehicle and each corner of the vehicle are unique and don't necessarily match other cars, even of the same model and model year. Based on those initial calibration numbers polled and stored by the module as your defaults values - so you can always go back to them if you get confused or wish to bring the car back to stock prior to a dealer visit - IIDTool calculates the theoretical maximum suspensions travel limit (in millimeters) that your specific car allows, which may therefore vary depending on your starting calibration numbers. Sometimes it's +/- 50mm, but usually it's a bit less. In the case of David's car, it's 43mm. Mine allows up to +46.
Highway running at -20mm (that's less than 1 inch) won't cause alignment issues unless you plan on doing it permanently, in which case you probably should get an alignment to correct the slightly modified geometry down there. Keep in mind that even the car's suspensions will eventually lower by 20mm on the highway (it's called High Speed mode), although you need to hit the ridiculous speed of 100mph before they will do so. I also find that lowering the car by as little as 20mm makes the vehicle handle better in turns / mountain roads, besides the slightly more aerodynamic stance on the highway (we are still talking about the aerodynamics of a brick, lol.)
