Your runs and Apple Health
My interval day shows fewer miles than I ran
On a hard day the planned distance counts the warm-up, the reps and the cool-down. The slow jogs between reps are never counted, so a session written as 3.49 miles is longer on your watch.
Your run’s own distance is always the whole thing. Only the planned figure leaves the rest jogs out, and the read says so under the whole run.