These figures are just statistics.
BMI regional fly to regional airports with tiny planes, it figures that they will be more punctual. They have less pax to get off and on.
It would also make sense that Ryanair are more puntual than BMI again because they often fly from secondary airports (in the case of PIK) where no one else operates. They also operate the 1 aircraft type and fly only to europe.
No airline above BMI operate into LHR therefore you would expect them to be more puntual.
Also it figures BMI are more puntual than BA as they operate less aircraft and routes, less things to cause delays.
No excuse for Easyjet though :-)