Hi Hoss,This is simple divide 40[mm] by 3 [x] for exit pupil and that would be 13.3 mm , if you are pretty young your eye pupil can open up to about 7mm in low light so about 6.3mm [13.3-7= 6.3] of light cone is wasted, your eye pupil can not take this in. Your eye pupil is stopping this down to 7 to 6 mm , it is like this scope is 3x by 21mm [with your eye pupil opened up to 7mm] . Also if you think about it, at 9x it is like you are closer to the object so with an exit pupil of 4.4mm [40mm divided by 9 = 4.4mm] and with 4mm being the minimum exit pupil for low light for the best of resolution and contrast to be able to see what you want, that is why you could possible see better with 9x instead of 3x in lower light. So that you know if you have 9 power by 63mm scope of similar quality you would most likely see better with it than your 3-9x40 scope in low light, and also better coatings (the Zeiss :-) , less lens like in fixed power can make a difference. BTW I sure as heck don't think I am an optics expert.:-)
Regards,Steve