Any contributing factor outside your base ability. Weather (too hot, too cold, just right), mental state (did someone put you on edge, sleep poorly), etc.

So some of those are more or less under your control. Maybe you can control your mental state better than me. Me, a particularly stressful day at work doesn't lend to the quiet mind I need for a run. Those are my 36 minute 5km days. But nicer weather, seeing the geese on the pond as they migrate, can put my mind at ease even on those stressful days and I might pull off one of my faster runs. I can't control that.