Why can this vary from one scene to another? Primarily due to the use of the dynamic irises. Each projector will handle a scene differently, depending on how bright the brightest areas are, as they analyze the scene and apply their algorithms to improve black levels.
This image below is the table scene from Aeon Flux. The HD803 does well in revealing dark details. Look to the various shadows cast (on the table), as well as the table top itself. Again, the HD803 as set up, is performing very well indeed!
One of more practical and best images I have been using is from Space Cowboys. I've been working with this slightly cropped image of the white suited astronaut (Clint Eastwood) next to part of the satellite. With a normal exposure, you can't see any details in the satellite on the left, but with the over exposed image, you can see what I see on the screen, when watching the Optoma HD803 home theater projector.
Note, of the three other projectors, all but the Epson, sell for at least $2000 more.
As you compare these, you'll be looking at all the details in the satellite. Note that the overall exposure varies a bit. For example, the HD803 doesn't reveal data that the JVC RS1 does, despite the RS1 exposure, overall being less overexposed.