Its how much buffer time each airline builds into the time table.
Just ran a quick check on expertflyer on the LHR-JFK route
AA115 Dep 8:30 Arr 11:00
DL4 Dep 8:30 Arr 11:30
BA117 Dep 8:55 Arr 11:20
VS3 Dep 9:30 Arr 12:10
DL gives the longest flight time and BA the shortest with a 35 min differential