I have had this same thing happen as a druid.
A shammy in a 25 man TOC was going on and on about his 4k healing how he was SO great. So after anub I post the healing meters to him(just him) just to tease him because I severely out healed him. And he goes on and on about how druids are hot healers and always heal more (druids main heal isn't a hot anymore, its a combination of hot with nourish and nourish being the top spell) So I show him that on the spell chart. Then he starts off on how all my heals were wasted because I was at like 46% overheal and he was a lot lower.
I try to explain how druids are now with wildgrowth being a random selection that heals for 25k overall, how dots tick even when they are at full health now (on the combat log) and inso skew the meters.
All this while I never got below half mana and he was asking for innervates.
People are just stupid is how I take it. They don't know how other classes work.
Priests will almost always have low overheal because their mending is a reactive heal.