I you are talking about their heal skill ups, the additional heals per skill up is not additive, they are multiplicative. So for example, a champion heals for 20% then a skill up of 5%, the calculation of their heal will be additional 5% of 20%, not 25%. So the added heal is sometimes very small to be noticeable. I hope this answers your problem.
I you are talking about their heal skill ups, the additional heals per skill up is not additive, they are multiplicative. So for example, a champion heals for 20% then a skill up of 5%, the calculation of their heal will be additional 5% of 20%, not 25%. So the added heal is sometimes very small to be noticeable. I hope this answers your problem.
Just to be extra clear - the books themselves are additive, and then multiplicative on the base. So if the base is 20%, and there are four booking levels for 5% apiece - the final result will be 20% * (1 + .05 + .05 + .05 + .05) = 24%.