Aren't lithium ion batteries micromanaged to within an inch of their lives by electronics within the computer and the battery? Doesn't whether your machine is plugged in or not have very little to do with what the charge current to the battery itself actually is? Plus, of course, there's no guarantee that when the battery meter reads 100% the battery is actually full --- mapping 100% in software to 80% on the battery seems like the easiest way to improve lifetime.

e.g. it's very obvious that Macbooks have multiple different charging regimes, and they use different ones depending on what the laptop's doing and the state of the battery.