That's because the sliders only go down to 1, not 0 (if you change the HTML and set the "min" value to 0 you can get to 0% or 100%). I guess it makes sense because none of the answers could possibly be 0 as far as I can see.

Or perhaps as an optimisation he could set the total to 0% or 100% immediately if certain answers are set to 0 - for example, if no user need the feature, it should be 0%, or if the time and cost is 0, it should be 100% (although that's absurd), etc.