I think it probably accounts for "likelihood of use within a distance" where the average person is likely to use an E-Bike for a 30 minutes ride but not a bike. So the regular bike only decreases your impact within a like 15 minute radius of travel.

So as an individual it would always be best to stretch the distances you go as far as possible with the least consuming mode, but as a population, the E-Bike prevents more driving/bus riding/ horse riding (?) than a bike does.