It’s always like this as maintaining maximum battery charge is worse for the battery.
Probably the battery manager kicked in if you play while plugged on the wall to preserve the battery.
And for extra info, almost every other device (especially phones) will just say 100% in this case to avoid confusion.
Because most batteries aren’t charged beyond 90% to reduce the electrode degradation
No they are charged to 100% but they stop charging when hitting 100% to preserve the battery
100% means charger voltage equals battery voltage and no more current is flowing. Unless your fast charging with something like 4.2 volts or 9v on some smart chargers, your average charger can’t really not stop at a 100 lol
When deck goes to 100%, if you’re still plugged in, at that moment the deck activates passthrough mode meaning it will cut off from drawing from the battery while it will only draw from the charging cable thus cutting a lot of the stress on said battery. It is perfectly normal, it’s a great feature