That is correct. Healing and shields are like damage - when it mentions a percentage increase when upgrading the skill, they really mean just that percentage, so a multiplicative increase and not an additive one. 

Also, rare champions just aren't going to do AOE heals of 40 percent of all allies' max HP. Even legendary healers like Sethallia and Shirimani do only 20 percent of allies' max HP on AOE. 

Really think they need to look at their wording as that deffinately isn't how it reads on paper.

I made the same mistake with Warpriest before I was informed of what it actually meant, and yes, it's badly misleading and makes the skill upgrade practically pointless (in the case of Warpriest, heal goes from 20% to either 24% or slightly higher depending on how the upgrades stack, at the cost of 3 books). Especially considering that there's also the +X% buff/debuff chance skill levels that, from what I've been told at least, is actually additive to the base chance.

I've always thought it goes "15% x (5% + 10% + 10%) = 18.75%" rather than "15% x 5% x 10% x 10% = 19%". Never tested it, though. Not a big difference, but still a difference.

Have you got the exact 19% in your tests?

your probably closer with oyur calculations TBH - i just did quick maths and got close to the numbers i was seeing

I understand your upset because the Healing part is tricky for people who are not use to it.

However, Abyssal is an amazing champion.

You shouldn't feel bad for leveling him up.

Abyssal can Farm Brutal 12-3 in like 7 seconds.

Abyssal is also helpful in Demon Spawn Faction.