Most Garmin watches allow you to get pace from Stryd while still getting distance from GPS. If the watch is set up that way, the source for distance and pace is different.

PowerCenter uses distance data to calculate average pace while the pace chart relies on Stryd pace data points. Since the source is different, there can be a deviation. Say that GPS is measuring more distance than Stryd would, the average pace based on GPS data would be lower than if it were based on Stryd data. Below is an example where the average pace calculated in PowerCenter is lower than any pace data point of the pace curve:

The workaround is to make sure that pace AND distance are from Stryd. In that case, the graph and average pace will match.