There are some options here:

- gear: there are people who can afford decent gear with right stats but there are also people who have to play with what they can get. Someone in perfect priest gear will always have better heals (in value not skill) than someone who plays in Graham with a piece or two from his scout main.

- acc: most people don't use them or look or pay for them till end game but those that have decent acc will have higher heals.

- stats: not everyone can afford to go full int/psy. Some have to stak some vit which has an immediate influence on healing

- skill: Last but not least there is skill and knowing what you are doing. As there are terrible scouts, defenders, ... there will be just bad priests who either lack knowledge or think they are the best but just suck and don't wang to take any advice. Block the last kind of players is my advice.