I'm not sure about this - as an illustration I have been the only healer on a PQ and done nothing but frantically heal a bunch of dps/tanks and come out on top of contribution.
Were there other healers on the pq? if there are this tends to put a big dent in your contributions as your competing to heal the same damage.
The other problem can be if people just arent taking much damage though I did find a dodgy way of working that by tanking a champion mob yourself and just surviving till others inevitably help you kill it
Conversely on a tough PQ with lots of damage taken and no other healers you should do very well.
I did have a question mark around hot's as opposed to straight heals for a while but they do seem to generate contribution.
Edit - with 16 people they probably werent taking much damage so you would do badly compared to those dealing damage.
My theory (Probably bullshit) an average of your contribution between all 3 stages is taken and then that is what is given to your chracter. So during boss stages, no one takes damage, okay what i mean there is Just your tank is, and you don't have group wide havok. So when all dps competitors are bashin out lots of damage, your doing no healing in comparison which boosts their average above yours.
Nah as a tank its possible to come in on the last stage and clobber the end boss a lot and end up top for contribution. If anything it is weighted towards the last stage as you get dish out more damage to a hero mob.
The big problem for healers is that the better/bigger the group doing the PQ the less damage is taken so healing gets progressively less contribution - thats a flaw in the system that I'm not sure they can balance.