Odd battery charge level behavior

iPad 10.2 2021 (the latest ipadOS 15.2(19C56) ) the apparent charge level rises significantly after unplugging the charger and restarting the iPad.

1- I connect 20W charger at 30% charge.

2- I shut down it at %65.

3- Right after shut-down I unplug the brick (the time passed between the last time I saw 65% charge and unplugging is not more than 10seconds).

4-I reopen it and "voila!" the charge level is 70%

It always happens like this. Proportional to last charge level I see the increase is higher (ie it rises from 60% to 62-63% or from 64%to 67-68%).

What the heck happening ? I mean, charge level measuring is so simple read the voltage level! Since charge level is linearly indicated by the voltage level of the battery. that's it.

Of course, under load, there is a little voltage drop but after taking the load into account, the correct charge level can be approximated very accurately especially for my new iPad and it's new battery.

Charging is done at room temp. 20-25 Celcius.

Lastly, I noticed iPad's logic board heats up significantly above 36 Celcius during charging - not the battery, it remains cold. (Im sure about the location thanks to teardown vids.)

It is very interesting because I haven't seen any similar behavior compared to my other devices ( Samsung tab s6 and other android phones.. their battery get heats up not).

So anyone has an idea? What is going on here?