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/fighter
