In 5e you don't get your full HD on level up. You get the average of the HD (or you roll for it, but that isn't implemented in BG3, at least not yet) plus your con modifier.
Your fighter has a d10 as their HD. The average result of a d10 roll is 5.5, rounded to 6. And 6+2 = 8 HP.
Similar for all your companions (e.g., Wizards get average of d6 which is 4, Rogues get average of d8 which is 5, etc)
