Engine warm, warm day, NOT the same as PS fluid hot.
Driving 12 miles on an 80 degree day isn't enough to get PS fluid "hot".
There's enough fluid in there to soak up quite a bit of heat during severe use. Severe use = constant turning of the wheel for an extended period, or powering a hydraulic winch.
As long as you're full to the cold mark when cold, you're fine. The fact that you don't reach the hot mark when hot just tells you that your usage is benign in comparison to what the engineers anticipated will heat the PS fluid.