Activity monitor not tracking workouts correctly

I have an Apple Watch series 4 (black stainless steel w/ gps and cell capability). I’ve noticed that when I do outside walking workout the time is not measured correctly in the activity app. If I walk for 30 mins, the activity app says I only walked for 23 mins! I know when I start the walking workout and I finish it says 30 mins. I only walk for 30 mins in the morning. 30 mins is the goal I set, but in the activity app the circle is not closed, and the data says I only walked for 23 mins. Last time this happened it said I only walked for 15 mins. Apparently the workout app and activity app aren’t synchronized. They were when I first started using them when I bought the watch about November of last year.