Sorry for posting so late but I just ran across your post. Perhaps my answer will help others should they run across this.
A number of laptop manufacturers include a manual setting either via software or via BIOS that caps the charging of a laptop battery at 80 percent. This is offered because only charging to 80% will increase the service life of the battery. The trade off is run-time.
A lot of times this feature goes by "Optimize for Battery Cycle Life" or something similar to that.
-John from www.SafeBatteries.com