I'm going to speculate so don't take this as confirmed at all.
If aoe heals work the same way as aoe dps you can only "hit" 5people at a time.
We know that 1hit != 1tag but let's just go with this basic idea for a second and exclude the actual amount of dps/hps requirement that comes after having "tagged" something.
This would mean that if you're running a "heal" build on a:
* d/d ele will tag 0.5625tag/sec.
* d/f ele will tag 0.5/sec
* s/d ele will tag 0.375/sec
* s/f ele will tag 0.25/sec
* stf ele will tag 15.5/sec
You can do the same thing for dealing damage but as you probably guessed you'll get a lot more tags by dealing damage.