Nunu Health Glitch When Hit Subtracted From Cap, Not Pool

Me and my friend (Sharkbait185 - Jhin) both noticed at the end of the game, a full tank loses its actual bonus HP when hit with Ashe's arrow. He leaves base with 4.2k 30 seconds prior, and seems like over time, the damaged was subtracted from the cap, not the pool, down to what I assume a lvl 15 "no HP item Nunu" would be. Was this just a visual glitch? The game ended so I wasnt able to observe if the overall HP went up if he ate a minion or went back to base. All the other players either didnt notice or didnt comment.

I'm assuming this is a visual glitch, similar to how junglers would sometimes show 4 cs at 15 minutes into the game depending on Fog Of War conditions (or something like that, i forget specifics) But since I was recording I made a short clip of it, and you can see his full build. I'll also post end game stats, if that helps any.

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2109166936/40116542

P.S This is my first post on these forums (I think) Hi Internet!