Good players know to focus fire, know how to dogged pursuit you, keep you stunned and interrupted. Bad players can still do plenty of damage- they just don't bother trying to stop a healer.
I understand how you are trying to logically show me your thought process, but it still doesn't make sense really. I guess if you are strictly a dps sorc, and you have absolutely no support from a single person on your team, and you are constantly being focused and not a single person is helping you in any way...then your numbers will suck. That is understandable and makes sense.
As a corruption hybrid though, my healing skyrockets when myself and teammates are being focused hard and properly. I'm taking way more damage and so are my teammates. I have to heal for a lot more, and even help with assisting on damage when playing a challenging game.
Something to think about is when I am being focused really hard, I always heal for a lot more because I have extra healing on myself from talents. I'm also constantly healing every second of the game if I am being focused religiously. Also good games last a lot longer than quick wins against bad players.
In a non-challenging game I will heal for 150-300k because there isn't much to be done. But in a challenging game I usually heal for 400-700k and do 75-125k damage.