<p>Just a chemistry note: the lithium ion batteries that all modern laptops (and electronics, for that matter) use do degrade if they are left at full charge. You should unplug your computer if you’re not using it for extended periods of time, and run the battery down once in a while.</p>
<p>As for your percentage issue, that is likely a glitch with the battery monitoring. It might read the voltage of the battery and compare it to what the voltage should be at various charge states. If the voltage is slightly low for whatever reason (which may or may not indicate a problem with your battery) then it might interpret that as battery at not-full charge.</p>