That's common for lots of batteries. My laptop has a setting to not charge between 50-70% because it lives on a dock and doesn't need max life in travel. Batteries are stored between 40 and 80% usually. So it makes sense that a car with the same battery chemistry recommends the same thing. It's only different in regards to a car being important in an emergency, but realistically, an emergency is unlikely to be both sudden and require long distance driving. So 100 miles of range is probably as good as 400 in common usage.