Mage Healing Mechanics on Allies

I'm currently a Healer Mage, and I've found out that while my healing on myself is consistent and based on my Hp as well as my Water Damage % which is around 7.1k 1st pulse (I have 36.8k hp and +121% water damage) my healings on allies isn't the same for every person. For some I heal just 7.4k hp, and for some others I heal up to 8.9k hp (1st pulse, they are significantly having lower hp than me, around 15-20k).
I'm not sure how my heals work on others, can you guys explain what influence my healing to allies and by how much? Thanks for reading, and I hope to get a reasonable answer.

 

The way I know is that the amount a mage heals is based on the other persons max health, but also your max HP + water dmg.