They use the average HP per level rule (there are two methods in 5e).
Fighter example :
Level 1 : 10 +Con
Level 2 : 6 + Con
Level 3 to 20 : 6 +Con each.
So what you are seeing is not incorrect. Both average (6 per level for Fighter) or roll (1d10) are proposed in the rules and used by D&D 5e players.
https://www.dndbeyond.com/classes/fighterAs I mentioned in my first post (you probably missed it) - I know this is an average of total hp pool per lvl.
The problem is that this information is nowhere to be found in the game.
On character creation menu - game informs you about MAX hp of your class / per lvl.
On the top of that (as I mentioned) - your enemies and NPC got max hp per lvl.
lvl 4 human paladin had over (around) 48hp, which is impossible even with 18 CON (I believe he had 14), when you apply the 'average' hp distribution on lvl 2,3,4.
