Healer performance is directly related to group composition far more than DPS is. I play a druid and when I compare the top parses, it’s against druids that healed a fight with no holy priest healer, one that died, or one that is extremely poor performing.
It would be far more accurate to compare healer parses against groups with a similar composition. DPS isn’t nearly as affected by group composition-- their fate is far more in their hands. The only time DPS HoT’s get screwed over is when a boss or add dies and their DoT’s haven’t finished. That doesn’t occur nearly as much as Druid HoT’s becoming overhealing. An above average parsing holy priest can (read: does) top up players before HoT’s have an opportunity to tick more than once or twice even though those players aren’t in any immediate danger.