Differences between pace monitoring and average pace

What was the total time and distance for this activity?

The time and distance coincide aprox with de pace average. Distance: 6390m and Time: 36:59
But the pace that the watch monitored and registered is between 5:00 - 5:20

36m59s for 6.390 km is an average pace of 5:47 min/km. 

If your instantaneous pace, while moving, remained around 5:20 min/km then it means that you were stopped for a total of 2m54s during the activity. 

Any time spent standing still counts towards the average pace calculation so long as you don’t pause/stop the activity. 

If you look the activity in Garmin Connect it should show you the Average Pace AND the Average Moving Pace. 

The average pace in motion is equal to the average pace. As you can see in the graph there is no time when the pace is less than 5:20 except for the first seconds (which I doubt that 15 seconds at 7 or 8 min/km will alter the average so much). It's not the first time it's happened to me.
Furthermore, when I published it on Strava, the average of each km was marked at 5:40 or more, and according to the watch, there has not been a single moment of the race that went at this pace.
If you look at the graph you will see that the average pace line is below the pace line and is not crossed for an instant. The average pace line should intersect with the pace line.

Are you using an HRM PRO or any other sensor that reports pace?

Two potential reasons:

1. Your actual pace was NOT 5:20, and the watch showed you 5:20 due to some inaccuracy. Are you using a sensor for pace (like a footpod or the HRM-Pro)?

2. Your actual pace was indeed 5:20, but for some reason (like for example problems with GPS reception) the distance is wrong, hence the incorrect average.

No, only my forerunner 955.

I don't use any external sensors, just the watch.
Today, for example, the graph makes more sense since the average pace marks it between the fastest and slowest pace.

I don't understand, there are days when it goes well and others it doesn't.