I have an observation I've made about doing this "extra charge" where you shut off the phone and charge more.
Seems to me that, if I turn on the phone again, after it's finished the 2nd charge, but still leave it plugged in, it drains down to the point where it was before that 2nd charge.
In other words, I charge with the phone on, unplug, power phone off, charge until it's green, turn on the phone, leave it plugged in for a while, unplug and I drop to 96% pretty quickly.
If I don't turn on the phone again until I take it off, it seems to hold the 100% longer.
Anyone notice this?