[GAMEPLAY] Possible item calculation bug?

Basically i was playing as a jungler. One specific item was this . The +25% actually show, at the time i first bought it, as 69 hp. i find it too little and tried to get more hp after that and did a calculation on my own. So basically my hp was 3874 and shows me i have 434 bonus hp 25% of it (3874/100)*25=950? i tried to -434 of the total hp i had to make sure i wasn't doing a miscalculation. So it would be (3440/100)*25=860? I am not sure whether it might be a bonus only received for health items after completion of this item? I find this particularly disturbing especially as a tanky jungler... Please have a look on the calculation or possibly make the item stats calculation more clear.