Light throttle and cruising throttle lean is good.
This is called running lean of peak.
You dont need a max power mixture when you are not making max power.
I see 16+ : 1 while cruising down the freeway at very light throttle.
I could get into a lot of technical discussion about about this, but
as long as you are runnning 10.5 -11:1 at WOT and the engine runs
and idles good while lean in light throttle I would not worry about this.
Democracy is two wolves and a sheep sitting down and voting on whats for lunch.