Have you been aiming the light at the wall and looking directly at the center of the reflection (the "hot spot") ? If you do that, a focused beam will be brighter than a more diffused one, even if the light that produces the diffused beam emits more light. I'm sure someone who is more knowledgeable and more suited for the job will chime in and explain the difference between lux and lumen, but in the meantime, you could try this: aim your lights at the ceiling, one at a a time, and try to estimate how well the entire room is lit up, or look at something in the corner of the room. That should give you a better sense of the total amount of light emitted.
You will not perceive the difference between 100 lumens and 280 lumens as you would the difference between driving your car at 100 / 280 kmph, but you should be able to see that the Zebralight is brighter if you do the "ceiling bounce", as it's often referred to here.
One more thing, if you want to get the most out of your Zebralight, you probably have to use NiMH cells rather than alkalines. A fresh alkaline should give you the max output for a short while though, but output will start to drop within minutes.