Please note that the original game is from 1993 and I ported and improved it. So some ingame decisions were not done by me and I can’t answer your question without guessing. If I have to guess, I would say because different professions are stronger than others. The warrior gets levels very slow but will be a machine in end game. Or maybe the designers thought that mastering as a warrior is harder than being a skilled adventurer. I can’t tell you for sure.
The values however should fit. But I understand the confusion. The tooltip is not accurate and I will fix this in the next release.
The real increase value is a random value between half the displayed value and the displayed value. So in your example you will get 5 to 10 points of LP for each level up. For SP and SLP your intelligence will also add a small bonus (INT divided by 25 to be precise).
As I said I will rework the tooltip to add this information. Thanks for reporting. ;)