Gaining health and mana points at a new level by Priests

[Chaosmetal]

After a little research, I found that Night Elves and High Elves gain 6 health points at level 3, while all other races gain 15 points. At the same time, Trolls and Tauren gain 10-11 mana points at levels 2 and 3, while all other races gain 24-26 points. What is this: Is this a bug or a hidden mechanic?
P.S. When I played as a Night Elf Priest at level 9, I received 31 mana points, but at level 10, I received 47 mana points, and at level 11, I received 18 mana points. Please explain what is happening. This increase in health and mana points is important for the Vagrant test. Thank you.