Well, most of the expert geeks say that all other smartphones these days have a circuit mechanism to cut off the connection of main supply with the battery when it gets fully charged. So, this means once you see the battery level on your smartphone at 100 percent then it is no longer on the main power supply or in other words you smartphone is back on its battery strength.
It is fine that the circuits cuts off the battery from the main supply when it reaches 100% but then it gets back to the power supply when it reaches 99%. So, this cycle of hopping between 99% and 100% creates a problem for your smartphone. Please note that it is not 99% percent for all the smartphones, few smartphones will plug your battery into the power supply once it reaches 90% and this figure varies as per different devices but the difference in these figures are not huge.
This process of frequent and continuous charging and discharging causes your smartphone to heat-up and thus hampering the strength of the battery of your smartphone.
Another point which can be noted that when your battery is between the level of 90% and 100%, then it produces a corrosive effect which further hampers the battery health. So, it is recommended that your smartphone should stay between these battery levels for as minimum time as it can be as the effect stated above is enhanced by the temperature elevation.
Just to let you know that usually there is steep fall from the battery level of 100% to 90% (once you take your smartphone out of the long charging period) but this is not visible on your smartphone because the manufacturers use a salty gauge which replaces this actual steep battery level fall by a slower or lesser steep fall.