The estimated battery life is wrong
When you check the remaining battery life, you may find that the time remaining that it reports is different to how long the battery actually lasts. This is because the amount of remaining battery life can only be estimated. Normally, the estimates improve over time.
In order to estimate the remaining battery life, a number of factors must be taken into account. One is the amount of power currently being used by the computer: power consumption varies depending on how many programs you have open, which devices are plugged in, and whether you are running any intensive tasks (like watching high-definition video or converting music files, for example). This changes from moment to moment, and is difficult to predict.
Another factor is how the battery discharges. Some batteries lose charge faster the emptier they get. Without precise knowledge of how the battery discharges, only a rough estimate of remaining battery life can be made.
As the battery discharges, the power manager will figure out its discharge properties and will learn how to make better estimates of battery life. They will never be completely accurate, though.
If you get a completely ridiculous battery life estimate (say, hundreds of days), the power manager is probably missing some of the data it needs to make a sensible estimate.
If you unplug the power and run the laptop on battery for a while, then plug it in and let it recharge again, the power manager should be able to get the data it needs.