I did actually read the footnotes, however as the average delay for each of the 3 airlines are different (i.e. each airlines punctuality stats have been taken into account) it can be concluded that of the low cost airlines, Bmibaby had the lowest average delay and consequently, the best on time performance.
Just for clarity:
Average Delay for bmi Regional: 6.2 Minutes
Average Delay for bmibaby: 7.1 minutes - Lowest of the low cost airlines
Average Delay for bmi (Mainline): 10.4 Minutes