Many laptops nowadays only charges the battery to 50% - 80% capacity unless you override to charge to 100%.
The basic theory is that the more often the battery is charged to 100%, the sooner the battery will die. This has to do with the chemistry inside the battery itself. You can say that the chemicals become less effective to retain a charge the more often you use it.
Not being able to replace the battery yourself kinda sux, but that's how Ultrabooks (and tablets) are designed.