The past 3 levels i've gotten 5 (not that i'm complaining), but I remember reading somewhere in the rules previously (I can't find the section detailing HP now) that it was determined randomly, 1-5 points.
“ 1. Upon creating a character: HP = round((5 + Race Bonus) * Race Multiplier) + 10 + Stat Points
2. When you reach a new XP level, you get: + (5 * Race Multiplier)
3. When you increase one of the above skills, you immediately get the HP bonus (+1 in strength means +1 HP).”