da* 16-50 f2.8 artificially restricted to f2.8?

I didn't use Pttl.  I used a flash with radio triggers set to 1/128th power and the camera on X mode (forces 1/180 shutter) f2.8 and iso 80.  The camera has no control over the exposure or the flash so there should be no inconsistency unless the flash is faulty and not able to deliver the same light each time.
I will do more test's with it tomorrow in my little studio with more lights and better backgrounds and tripods and flash meter but really..  no tripod, shiny door and the like shouldn't really effect the test that much as the flash and the door never moved so no mater where I shot it from it would still have gotten the same amount of light and I know angle of light can make things appear brighter/darker but I was behind the flash (and sitting on the same spot) for each shot taken.