The "State of Charge" isn't really the SoC, it's the voltage across the battery terminals. (The true SoC can only be determined by taking the battery apart and chemically analyzing it - which also destroys it. Measuring the voltage is the best we can do - and it's an estimate, based on how we know lithium batteries work.) When the charger is connected, the terminal voltage on the battery is higher, so when it says 100% on the status bar, it's not yet fully charged. The phone waits until the voltage gets to the "100% with the charger attached" point before telling you that it's fully charged. (At which point, charging stops, so you don't have to yank the charger out. Leaving it plugged in for another month won't do any damage.)
If you unplug the charger the moment the status bar hits 100%, you'll probably notice the indicator drop to 99% very quickly. That's because that was the true SoC.
(BTW, what email app are you running? I'm running the same one, but I can't find a link to it anywhere, or any reference to it in the phone, except the icon.)