This lens ramps at any aperture, meaning that it will loose one full stop at 100mm.
Honestly, I've played a bit with it at IBC and I could not tell the loss of light zooming in and out from 21 to 100mm.
What stop were you at when you were using it.. to see no effect.. its usually pretty noticeable wide open.. well on zooms that ramp at the end.. I guess its cheaper to make with a gradual ramp through out the whole range.. ?.. but surely you would see something..