Uncle didn't really address this directly but it is part of his solution in a way... and it definitely matters: If you add % max hp to a unit via a trigger like this and want to later remove that bonus hp you have to keep track of exactly how much hp you add to undo it. Why? Couple ways it can go wrong if you don't.
If you add x% hp, when you go to remove it and take away x% of the new total, you'll remove more than you added! Consider the simple example of adding 10% bonus hp to a unit with 200 hp. 200 x 1.10 = 220. Removing 10: 220 x 0.9 = 198. The unit lost max hp overall.
Technically you can do some arithmetic to end up back at the correct original number. If A x B = C, then A = C/B. For the example with a 10% bonus to 200 base you would compute the 'original' hp as: 220/1.10 = 200. This would be a perfectly fine thing to do if there is no other way for anything to affect total hp of a unit in your map. If there is any other way a units HP could change (increase or decrease) this method will not account for this and can improperly reduce the max hp when units leave the aura. Consider that unit upgrades, items with strength or hp, heroes leveling up, the ability to morph into a different unit type with a different max hp are all things that have the potential to break this.
Again the example of a 200 hp unit with a 10% bonus but this time assume it picks up an item that grants +50 hp while inside of the aura, and then walks away. It should have 250 hp afterward but does it? 200 x 1.10 + 50 = 270, then 270/1.10 = 245.5! Some of the +50 from the item was lost when the unit left the aura. Also consider having the item before entering the aura, then dropping it while inside the aura: 250 x 1.10 = 275, then it drops the item so 225/1.10 = 204.5. The unit would gain total hp permanently this way! If the unit picked up the item before entering the aura and never dropped it then these problems would not occur.
The only surefire way to avoid this potential issue is to calculate the bonus hp given to the unit, store that exact number in a real variable, and when you need to 'undo' the aura remove exactly that much HP again. This is what Uncle is using the Celerity_MaxHP[Celerity_CV] variable for in his example.