Without WAAS capability or WAAS coverage for the GPS (pretty sure the LR3 does not have it), the vertical accuracy is only good to a few hundred feet. WAAS is a FAA system that adds vertical accuracy for GPS approaches for use with aircraft, but there are land based GPS systems that have it now. Still, the barometric altimeters are the best. Just be sure to calibrate them with the current barometric pressure for an area.
4406ft is the highest point in Scotland (figuring that is also the highest for the whole of the UK), so maybe LR figured mountains as high as ours here in America (which are short compared to those in Asia or South America) are only for climbing. Dunno, Alps are pretty high too. I agree it is pretty ridiculous to have a maximum altitude reading, but also, I don't see how it matters that much in a land vehicle.