The aerodynamics are from a roll-down test done by the manufacturer. They do a curve-fit, ending up with 3 parameters (A,B,C) that they send to the EPA. That plus actual weight is all the dyno needs. I think the 'set' parameters are to compensate for the roll-down on the dyno itself.
You can see these on the detailed test data at
this EPA page. Obviously this doesn't take into account hills, but I think that's why the tests speed up and slow down so much... it's like going over hills.
The only question is: what exactly are A, B, and C? They're never really defined by the EPA. Edit: see below for an improved guess at the formula.