I claim no expertise on projector lighting, just a long time ardent photography buff, having dealt a lot with f-stops on camera lenses. I got the following definition from Answers.com:
"The greater the f-number, the less light per unit area reaches the image plane of the system."
Since the f-number at the longer end of the zoom range is larger, less light will reach the image plane (screen) when the projector is farther away from the screen, if the screen size remains constant. I'm not sure, but the effect may be linear (eg. at half the zoom, the f-number could be mid way between 3.2 and 4.3). If anyone has a better understanding of this, I would appreciate their input.
The sony reference I made was to some posts on other threads regarding the Ruby. The consensus seems to be that the projector is brightest at its widest zoom. You can check the specs on the Ruby for the f-numbers on the Ruby lens, maybe that will help to determine how much difference we should expect on the JVC.
Whoops, sorry Cam Man, I posted before I saw your reply
Sony Ruby ( from Sony web site) fl 8.7 to 33.7 mm/F2.54 to 3.53 mm