Can anyone explain how my dragonair's health went up by 1hp when I gave it a protein in Pokemon Heartgold?

Question

I've been ev training my dragonair in Pokemon heartgold. I've decided to give it a protein and I noticed at the same time when I gave it to it that the health also went up by 1. I was just wondering if anyone had an explanation as to why this happened? I'm not complaining or anything lol. Just wanted to know how that could've happened. Oh and by the way, not sure if this helps but it's about 232 exp away from it's next level when I gave it the protein. Do the ev's possibly all add up and raise the stats from training when you give it a vitamin without leveling up? I don't get it lol

Answer 1

Stats are also recalculated immediately if a Vitamin or stat-reducing Berry (see below) is used on the Pokémon.

source
So the most likely explanation is that Dragonair gained a bunch of HP EVs since it last leveled up, but the EVs haven't started affecting its HP stat yet. When you use a protein, it recalculates all of Dragonair's stats, including HP, so the new EVs get to raise the HP stat.