The PR graph follows your highest average power (or NP? whatever) per period of time, not your overall average at a point in time during your ride. Here’s an example from one of my rides last week:
For that ride, my highest average power over any 11-minute period was 113 watts. You can see the highlighted bit in the graph is one interval, plus the last rest interval in the warm-up.
If you push out to 17 minutes the time period can include two full intervals, which pushes the average for that period up to 123.