I see it as a experience and veterancy thing. And it can go both ways if you want it to. Hell there can even be some debate in setting between orders of all kinds from religious to secular doctors about the best way to be a healer.

You start out as ether a heavily armored knight type healer or a robed weaking healer and as you level up and gain more experience you switch to robes or armor depending on your orders belief's. If you start out as a robe and then start waring armor after you get stronger that is because your order feels the higher up in your orders hierarchy you get and the more dangerous the missions you have to take you will need more and more protections to survive the oncoming dangers coming for you.

The orders that start with heavy armor and switch to robes do so because the armor interferes with stronger spell casting. And the stronger you get with white magic the less you need weapons and armor to protect yourself with. To the point you are just holding up your hands and projecting light shields and blasting fools with laser beams to protect yourself and big heavy armor and weapons would get in the way of that. When all you need at the end of the day is a holy staff and the sacred robes of your armor. Wearing heavy armor at that high point in your career is seen as being a bit cowardly and weak of will.

And as always other orders and careers will have there own way of doing things that they think make for the best healers of the sick and injured.