Ranger: Double mana cost on minor healing

[Valanek]

I'm not sure if this is a bug or a feature I'm unaware of, but when my ranger casts Minor Healing (not high enough for other healing yet) it sometimes costs double the mana. Does anybody happen to know why?

EDIT: Just got Light Healing and it's doing the same thing. I was mistaken thinking it was double, it's occasionally adding 10 mana to the cost (Minor Healing 10 -> 20mana, Light Healing 25 -> 35 mana).