I have been thinking about why your exposure is so different from mine.
Looking at the QE graphs for both cameras and reading about it, i think mine might be a little more sensitive at the UV end of the spectrum.
I copied this off their website, it mentions a UV-IR blocking filter?
ALTAIRGP130M-FULL Package:
CS-Mount adapter with built-in Clear Optical Window, AR coatings. (Attached at factory but removable).
CS-Mount adapter with built-in UV-IR Blocking Filter, AR coated. (Removable)
I also wonder whether your scope has some sort coating on the mirror perhaps, not been able to find any info despite a long look online.
Not sure i have helped, but i was interesting reading about the subject anyway.