the calculator seems flawed. Yesterday I used it to compare the image brightness of my projector on 3 potential screens measuring 119", 110", and 100". The calculator indicated the brightness for the 3 screens (using same projector distance and screen gain) would be 17 fL, 17 fL, and 19 fL, respectively.
This did not seem right to me, so I did the math on my own and determined the actual image brightness would most likely be 14.8 fL, 17.2 fL, and 21 fL. This makes sense to me, and casts doubt on the accuracy of that calculator.
Of course, in your case, the calculator could be using different lumen values based on different picture modes for each projector (e.g. assuming the brightest mode for the InFocus and a dimmer mode for the BenQ.) My projector is a Panasonic PT-AE8000, which is rated at 2400 lumens, but the ProjectorCentral calculator seems to be using a lumen value of about 800 for its calculations, which is certainly closer to a real-world value. One flaw of the calculator is that it does not reveal what lumen level it is using in its calculations.