which ROM and kernel do you use?
Possibly some thirdparty apps may cause reboot.
The increased percentage of battery after reboot is not actual %, you might have noticed that the increased percentage drains faster.
The changed percentage after reboot is possibly due to some changes in the battery stats calculation (during approximation of remaining battery). It happens with me also. But as I said in the first post that my battery remains same overnight, but in this case when battery goes up after restart then my battery drains overnight to its actual level. So don't worry about it.
used greenify and battery doctor to make sure it's not apps related
reboot under intensive gpu use, such as games and youtubes
but random, at other times i can game all the way to 1%
full charge and thru 6 hours sleep, i'll be left with 50-60% battery