Battery Health is an estimation based on an algorithm. The thing is, that it is based on the capacity of the battery from the reported capacity. An article I was reading in a trade magazine says that the capacity is coded into the software, but the actual capacity of the battery could be higher than the reported capacity when it is manufactured. With that in mind, even after it has been used for a period of time and should show a decrease in health, while it has decreased, it still shows the 100% because the initial capacity was actually higher than the number used to calculate the percentage so mathematically it is still over 100%. It cannot report over 100%, so there is really no way of knowing what the actual capacity was when new.
I wouldn't worry about the battery unless you begin to have issues with holding a charge, and then you can get Apple to do a remote diagnostic on the battery to see what the capacity is. If you are really concerned and have an Apple Store close by, make a Genius Bar appointment and discuss this with them and they can also run a diagnostic there.
