Josh, this is very useful data. I've talk before about stones heating up a bit faster in the lower quadrant of an oven, but, because the thermostat is cycling the oven on and off, there's going to be an equilibrium reached, which your numbers seem to show. Because the heat source is on the bottom and there's no deflection involved, there will always be a slight discrepancy between upper and lower temperatures, but, if given enough time to fully pre-heat, not only will the discrepancy reduce, but you should see a similar final stone/steel temp in all oven shelf positions.
Clearance plays a role- the more flow you have from bottom to top, the faster the equilibrium takes place. In a broilerless setup, I take advantage of this and put in a false ceiling with a secondary ceiling above that in an attempt to keep the heat in the bottom, but if you have a broiler in the main compartment, you don't have to worry about that.
As to your question, I don't think anything particularly special is going on here other than your oven is running a bit hot. An IR thermometer would definitely help to answer some questions, but, until you obtain one, I'd put the steel towards the top of the oven (5" from the broiler) and pre-heat if for 60 minutes on 525 and see what kind of color you're getting at 4 minutes. If it's still too charred, go with 500.
Btw, this is a pretty good deal:http://www.amazon.com/Etekcity-Temperature-Non-Contact-Infrared-Thermometer/dp/B00837ZGRY/?tag=pizzamaking-20
It only goes to 716, but that's fine for NY style. I swear the 1300F model was $22 a few days ago, but now it's $25. That's Amazon for you. If you think you might end up ever doing Neapolitan in a WFO, I'd splurge for the higher temp.