When you hover over the item in game it shows the base amount of health (920) and a section with the bonus health (+4-450). If it were included like you say, that would be an immediate downgrade in health from the items that went into creating it. 920-450=470. Giant's belt is 430 and the ruby thing-a-majig is 180. You don't go down in stats when you build up an item. Sure, sometimes random bonus stats get lost (like gold per 10 and whatnot), but you don't instantly dump stats.

I think what it means when it says "included in the item's stats" is that it is counted in the item totals at the top of the page. When creating the page, they were. Now they aren't, so it would appear there is a bug with the display of Volibear's health pools. I imagine they are still working out the bugs on his base stats.

At the end there you say Warmog's gives 1400 health when maxed. On warmogs tooltip it says in the end "Bonuses cap at +450 health, and +15 health Regen per 5. (Already included in item statistics)". I reckon this means you get 920 health from wmogs when maxed.