I don't get the exact same distance every time for exact same route.
For example, went to one of the nearby coffee shops (5.6-ish miles round trip) the other day. Always go same route. YTD, most distances agree (three times same [including with newest Watch OS], two are +/- 0.01 miles), but did get a big swing a month ago (+0.04).
But at that scale, we're talking swings of 58-233 feet. Pretty minor, imo.
If using autostart of workouts, can maybe seeing it triggering a little later than you are used to (don't use that myself). Or might be getting more accurate results now.
If really concerned, and if you can access an outdoor running track, do four laps on that. Will be either 1760 yards or 1600 meters and then can see how off the Watch might be.