Intervals on Workout App, WatchOS 9

Greetings peterfromwoodbury,

For unexpected behavior when running, begin by calibrating your Apple Watch for the best workout results:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

"Calibrate your Apple Watch

1. While wearing your Apple Watch, go to a flat, open outdoor area that offers good GPS reception and clear skies. 
2. If you have Apple Watch Series 2 or later, you just need your Apple Watch. If you have Apple Watch Series 1 or earlier, bring your iPhone for GPS. Hold your iPhone in your hand, or wear it on an armband or waistband.
3. Open the Workout app. To start, tap Outdoor Walk or Outdoor Run. To set a goal before starting, tap .
4. Walk or run at your normal pace for about 20 minutes.

If you don't have time to finish the workout, you can complete 20 minutes over multiple outdoor Workout sessions. If you work out at different speeds, you should also calibrate for 20 minutes at each of the speeds that you walk or run.

Whenever you walk or run outside using the above steps, your Apple Watch continues to calibrate the accelerometer by learning your stride length at different speeds. Calibration can also improve the accuracy of your calorie calculations in the Workout app, and the calorie, distance, Move, and Exercise calculations in the Activity app."

Take care.

There seems to be an bug in the announcement. I get and other here have reported getting the time/average in the second announcement. There can be a little difference between the to "averages" but really you are just getting the average/the time for the last mile like you are describing.

As you found out the second announcement is really just the first "average" with a little error. The error is like it adds a second to the first time like it read the clock again for the second measurement and 1 second passed between them.