What affects how much life I get per point of Vitality?

I know when I started my character, I was getting 10 life per point of Vitality.

However, looking at my level 41 Barbarian, I see he's now getting 16 life per point of Vitality. As near as I can tell, this change isn't caused by any of my gear or skills.

What determines how much life you get per point of vitality? If it's level based, when does the number go up? Is it the same for all classes?

I'm currently a level 46 Wizard, and I get 21 life per point. My templar is level 45, and gets 20 life per point. However, @OrigamiRobot's is 10 per point at level 35, and @StrixVaria's is 29 at level 54.

As far as I can see, none of my equipment affects this - it is the same if I am naked. Also, I don't think class has anything to do with it.

It appears to be linear - using yours, mine, my templar's, and @OrigamiRobot's life per vitality as data points, it could be Life Per Vitality = 10 + (level - 35), with a minimum of 10.

EDIT: Confirmed - @OrigamiRobot's life per vitality at level 36 is 11, so unless something really wacky is going on, that's your formula.