You see... the Rocket was originally designed to be a 3L four cylinder. However, that much torque and horsepower would just strip the rear tire off and cause every adult female within a thousand yards to spontaneously ovulate. So, in proper British tradition, rather than retool, they just bandsaw that fourth cylinder off right before assembly (rumors that every third Rocket was assembled from the leftover "short ends" are completely false) and leave the plug hanging. This makes the rear tire last longer and greatly reduces the birth rate amongst unwed boogans. Sadly, a decade of this population reduction has severely impacted Harley Davidson sales.
Or, you know, what R-III-R Turbo said.