Fully charged on the icon is actually only 80% of actual capacity, and empty is 40%. The reason you rarely see the battery completely fill up is because it tries to manage it so that there's always a bit of space available for more regen, the vehicle can't predict what's coming up. Sometimes you can fill the battery by going down a very long, steep hill, this is a common thing around here, but if you don't do that kind of hill then you probably won't see it too often, if ever. The other reason for it rarely filling all the way up is that there are varying amounts of battery charge for each 'bar' on the screen.
see:
http://privatenrg.com/
Also, keep in mind that there's nothing wrong with it never being full. That is perfectly normal and healthy operation.