My personal experience with iPhones is that they report battery % at a higher level than it's true charge. For the past 3-4 years with the latest model iPhone each year, I have used an ammeter in my charging circuit. When the phone display meter hits 100%, the charge current drops gradually from 1.6 amps at 95% to around .8-1A at 100%. It then tapers off slowly for about 30-45 minutes to .5A or so. This trickle will power the screen (Set to 50% brightness per Siri), cellular, gps, etc. continuously until removed from the wireless stand. I suspect this is for several reasons, but mostly so that people don't have to wait as long to hit the "Full 100%" charge level before unplugging, and also to protect the battery to some degree I suppose.

Here is a link to an updated meter from Amazon I'm using. Mine is older, and does not have the mode switch and only reads Volt, Amps.