coming from a background in public-safety communications systems, there is a very distinct problem with the "battery meter" on MOST electronic devices.
quite often, the "Battery Meter" is a gauge based on Voltage, rather than Amperage. Since voltage gradually decreases, although not in a linear fashion, as the voltage gets low, the battery dies quicker. whereas amperage does decreases in a more linear fashion.
to see this best... take a glass of water that is full. that will be our battery... poke a hole into the very bottom of the cup and watch the water flow out...
the force of the flowing water is voltage, the volume of water is amperage.
as the water drops to the bottom of the cup, the force of water decreases, and eventually trails off. but the force slows down as the water level in the cup drops. THUS the voltage drops, and the meter shows to the best of its ability show the "level" of the battery.
there are a few battery technologies which show the actual level of the battery, but the battery is much larger than original batteries, and the chargers get very large and complex too. There is a technology used by Motorola called by Motorola, IMPRES, whereby the charger working with the battery, actually measures how much amperage goes in and out of a battery during charging and use. those batteries are most probably the best check of actually battery capacity.
Apple DOES use similar technology in their Laptops, see MacBook / MacBook Pro: About This Mac --> More info... --> Power --> Charge Information will show Charge Remaining, and Capacity, both are mAh measurements, or milliAmperage Hours
hope that helps..