Basically, extra skill points. I’m 99% sure this isn’t intended, but if it is, that would be a separate bug, since it’s only available once per session.
The attachment shows a character with 44 skill points. My character also has an exp bar although it’s clearly level 40. When that bar fills up again, I’ll get another skill point. (Before anyone points out that my build is bad for not having armor pierce, I was testing archer builds against the sheep level.)
To reproduce, you just have to reload the save file. Your characters who are level 40 will automatically start getting EXP from each stage you do until they level. Everytime a level 40 levels within a single session, they stop gaining exp until the next time you reload a save file, in which you’re given the opportunity for another skill point.