Priests Actively with Spirit shell and Power Word: Shield. as well as passivly with Divine Aegis
as well as Paladins Passively with Illuminated healing (~30% of healing into a shield in TOT Gear)
this allows these 2 classes to effectively PREEMPTIVELY heal incoming damage in the case of the priest both actively with shields and passively with their mastery. allowing them to heal incoming damage before it actually hits. leading to the other benefit of preemptive healing
Increasing the raids effective health.
someone who has 500k hp and a 100k shield effectively has 120% hp. allowing them to survive where other healers cannot.
if a group has 500k hp and is going to take a 500k damage hit only 3 of the 5 healing classes can keep that group alive. Priests and paladins by preemptively healing the group with bubbles and shamans via ancestral vigor who can increase the groups effective health by literally increasing the groups max health with Ancestral vigor. which leads to the other 2 healing classes being generally inferior in most PVE situations.
druids and monks have little to no ability to prepare a group/raid for incoming damage. they can only heal the raid after the damage hits. leading to a further advantage of preemptive healing.
as healers get better gear and absorbs become a larger portion of healing done, classes without absorbs will perform less favorably druids/monks/shamans can only heal damage that is taken. i cannot describe how annoying and disheartening it is to see a massive 100k+ aoe damage hit the raid, no one's healthbars move and the 2 priests and paladins rocket up the healing done chart. and as they become better geared that disparity will only become greater.
overall the output of the healing classes seems fairly similar with shamans being low but bringing extremely valuable raid cooldowns/ AV however the fact that 2 classes can heal before damage is taken gives them a definite advantage in todays raiding environment.
Absorbs are Very powerful but only available to priests and paladins
Absorbs allow classes to heal before other classes
As people get more geared this disparity will become more pronounced as a larger portion of incoming damage is absorbed instead of healed.