Soraka Tooltip vs Actual Heal

Just played Soraka and the amount I was healing my teamates for was significantly lower than what the tool tip says. Tool tip reads: "ACTIVE: Soraka blesses a friendly unit, restoring its health. Astral Infusion cannot be self-cast, nor can it be cast if Soraka is below 5% of her maximum health." Heals for 120/150/180/210/240 (+60% ap). At rank 1 I was healing for only 77 where given the tool tip any my current amount of AP should have been almost 140. I didn't do the math as I ranked up but it seemed like it was giving less and less percentage wise as I ranked up.

Nowhere in the skill explanation does it say it is dependent on the health level of your ally or yourself. It says it gives a flat heal rate and that amount given doesn't change based on their missing health as someone in my game suggested it might.

Anybody know why? Is the tool tip just wrong?