Basically some airlines exaggerate their schedules thus if they depart late they can arrive on time, Ryanair do this all the time to retain their puncuality record whilst other airlines may 'undercut' their schedules to get more blood out of the same stone.
They all fly the same airways and the same routes but obviously different aircraft types have different cruising speeds, there are weekend only airways, when the military are not active, that can shave a few minutes off a schedule but what happens in reality, at quieter times, is that ATC will give direct point to point routings, sod the airways, and this will shave even further minutes off the shedule.