Experience tells us that pushing back is not the same as departure even if that is the metric that is used. Recently I flew SNA-SFO (commuter jet). We pushed back, taxied a few feet, and then shut down for 45 minutes. The UA app credited the flight with a departure and predicted an arrival time based on the supposed departure, but we sat on the apron instead. There was an issue of flow control into SFO, so the delay wasn't UA's fault. But this illustrates that "on time departures" ain't necessarily so.