That would be bump charging.
From what I've gathered, phones charge to 100% and then drop. Once they hit around 95%, they start charging again. This is because it can dammage your battery and shorten its overall life if you are constantly getting power to your device. It's just a safety precaution.
If seeing 100% is that important, charge it until it says charged, then unplug it, turn it off, and plug it back in. Leave it plugged in for a few minutes and power it back up. If it doesn't say 100% then, plug it in one more time and will almost assuredly hit 100% within a few minutes.
To be quite honest, though, I doubt you'll get a whole lot more out of that last 1-2% of your battery.