I guess it's kinda like my friends Honda Civic. For some reason, sometimes it just seems to gain gas over time. Hey, your battery was prob made in Japan too. Maybe the Japanese just know how to break the laws of thermodynamics. Either that or it's just a normal "bug" that you shouldn't worry about. I'm voting for the former.
Look at mine. I got my battery replaced at the beginning of May because the battery update screwed up my first one. Even on that one I was still at 6000mAh after 11 months. I will be using the battery every day now that fall classes have started again so I hope my battery stays the same. The red dot is showing what it is currently at.
I figure that Apple is simply shipping batteries with different capacities, ranging between the advertised 5500mAh and 6400mAh. There can be any number of reasons for this, like they are buying buying batteries from different producers simultaneously, or are in the process of clearing stocks of older batteries no longer in production.
You just got lucky and received a unit with higher capacity than advertised.