First post around here. *Yay*
Rad, I think you can attribute both the rising response and the high sensitivity up top to directivity. Under sane and normal circumstances, the highest sensitivity you can reach in half space is 105-106db. Of course, in quarter space, you can add another 6 to that. The bigger the baffle is in relation to the wavelength, the more the source will be seeing a quarter space scenario. (Then ofc you might be able to see another db of sensitivity here or there, caused by diffraction off of the edges of the baffle.)
This right here is a pretty neat little piece of software: http://www.tolvan.com/edge/help.htm
It calculates the gain caused by the baffle. (It also does calculate what kind of a passive crossover you'd need to compensate for the baffle gain.)
Toying around with that software(doubling the height of the cab to emulate a half space scenario), I find that the baffle adds about 1db at 30hz, rising to a 4,5db addition at 100hz. That should explain most of the phenomenon.