I think the tank healing amount is mostly due to the monster's level but also feel the rarity of the card has a part of it as well.

The battle I shared had the Equalizer rule-set. My opponent's monsters all had max 15 health and mine had 14. Their tank heal was a level 6 common Wood Nymph. Mine was a level 3 legendary Corrupted Pegasus.

In rounds 2-4 & 6, their Wood Nymph healed their Peaceful Giant tank +5. In rounds 5 & 7, it healed their tank +4. (The Wood Nymph died in the 8th round.)

In all of the rounds of the battle aside from the first one where my Furious Chicken only took 3 damage, my Corrupted Pegasus healed my tank chicken (love typing that) +4.

Curious, I battled this morning and noticed that my Corrupted Pegasus was only healing my tanks +3. I haven't had the Equalizer rule-set again so I thought maybe that was why it had healed more.

Then I noticed fluctuations in tank healing amounts with other monsters. Sometimes my Level 5 rare Beetle Queen healed +3 and others +4 (and those weren't with the Equalizer so it wasn't the rule-set making the difference).

Using my level 1 common Wood Nymph in battle showed the same variance. Sometimes healing +2 and sometimes +3. I think there is some kind of programmed randomized percentage involved.

A little while ago I lost a battle that had a level 6 common Crustacean King who was healing +6 and +5. Got my ass handed to me. It's totally worth it to level up those healers. ;^)