It does that from time to time. I think you typically have to let the capacity drop to under 95% before it will allow a full charge back to 100%.
http://manuals.info.apple.com/en/MacBook13inchUsersGuide.pdf
If the battery icon in the Finder menu bar is set to show the percentage of charge, sometimes the battery won’t show a full 100 percent charge when the power adapter is attached. This is normal behavior; battery life is maximized if charging is not continuously cycled on and off when the battery’s charge capacity is between 95 and 100 percent. When the battery level eventually drops below 95 percent, it will charge all the way up to 100 percent.
I've heard that lithium rechargeable batteries can last considerably longer if the batteries are never charged to no more than 90% of capacity. Most rechargeable battery chemistries (save lead-acid) lose a little bit of capacity when charged to near maximum and/or drained to near empty charge. Hybrid car batteries can typically last the car's lifespan because the batteries are intentionally kept between 40% to 75% of capacity. The lithium polymer battery might lose 20% of its capacity after 500 full charge-discharge cycles. A NiMH hybrid car battery pack can last over 200,000 partial discharge cycles.
I wish there was a setting for maximum battery life (wouldn't have to be as extreme as a hybrid car battery), although it doesn't match the way most people use their computers.
