Sounds like they're using standard 5e fixed HP gains. So, for a fighter (d10) with a CON of 10 (+0), you get:
Level 1: Max on class die (10) + CON modifier (0) = 10
Level 2: Average on class die, rounded up (5.5-->6) + CON modifier (0) = 6 + 10 = 16
Level 3: Average on class die, rounded up (5.5-->6) + CON modifier (0) = 6 + 16 = 22