As others have stated, those comparisons on which sports burn more calories are slightly flawed in they they typically presume ideal conditions. If ideal conditions are the case then yes, typically running is a more "efficient" workout, if by efficient you mean maximum calorie burn in least amount of time. However, when those figures are computed for cycling it is typically on flats with the rider going at a maintainable speed. Now I can ride for some time at 12-15mph on pure flats without really thinking about it, but if I start adding hills and/or sprints into the mix my heart rate increases rapidly and I start to expend energy much more quickly.