I've seen that on other devices also. It seems that there's a line at 'charged enough' that is just a little bit below 100% charged.
I presume some engineer somewhere decided that charging to 97% (or whatever) is a sufficient charge to not cause the battery to fail prematurely (which I'm led to believe that is less of an issue with these batteries anyway).
I've noticed that the last 3% takes a long time, a trickle charge as Dave noted, so there is probably some of "Let's turn the light green at 97% so the users won't complain about how long it takes to charge the battery" going on also. Or, that could just be me being old and jaded.