Traffic patterns also matter. I see a significantly shorter wait and fewer stops when school is not in session. And yes, accessory use such as running the heater and using the lights can account for 5-8% hit on economy alone.
As an experiment, I idled our 5 in the driveway with the Scanguage viewing the LOD display (which is an aggregated arbitrary computation of fuel demand based on things like throttle position and injector pulse width). I found that turning on the lights, stereo, and heater fan without the AC accounted for an over 30% hit on idle load. In a 40 mph zone that dropped to 5-10% difference.
I think the last fill was short, and I adjusted the ScanGauge to match; perhaps that's part of what I'm observing.
I don't normally encounter schoolbuses (despite passing a school), but today I did and still managed one of my best morning commutes ever. It was also 61 degrees and rainy! Maybe it IS traffic, maybe everybody is on vacation this week or something.
Another factor is the lack of wife as passenger. I can drive more efficiently without worrying about a passenger's comfort, and I can concentrate on driving more with my music up and no wife to talk to.
These factors all coincided with the tank I filled just before a few warm, sunny days. Neato.
This is a topic of interest and I'd like to hear more. unfortunately I don't have any hard data to add to the topic and then theories that have already been stated.
I do find it very strange that your iat and ect temps where higher with a grill block and your getting worse mileage. This is just a guess, but the EMS could be doing something weird with the timing and or fuel because of the abnormally high temps with the grill block. There could have been some mild inaudible knock that the computer picked up and compensated for. or a fail safe if the ect get to high. a lot of the really specific functions of any particular EMS really aren't published in say all data or Mitchell. I haven't seen any real in depth info on how the computer is programmed on cars.