These battery stats on my 6a has been... interesting...
I've been using the 6a since the start of this year, and alongside it Accubattery to remind me to unplug my phone whenever it reaches 85% charge.
I know Accubattery stats are just an approximation, and I've only been using this phone for half a year, but still quite amusing to see the battery health go up instead of dropping.
You should let the battery go to zero and then charge to 100% from time to time, even though it's stressful to the battery.
Accubattery recommends once a month to avoid these kind of odd values I believe.