[Arena]Imp Devotion Aura vs Holy Guidance

I see high rated holy palys prefer to spec 52/19/00 with an additional point in holy guidance instead of 51/20/00 with one point in imp devotion aura and I'm wondering why.
To my calculations 2% more heal from imp devo is a bigger increasement to the paladins healing spells than the extra spellpower from holy guidance.
In addition the arena partners do also 2% more heal. For expl a warrior should do 2% more heal with second wind and blood craze.
Do I miss smth?