If you zoom in closely you will see why this is mathematically possible:
This is the 3:45 interval with an average power of 269, and you can view it for yourself here.
In this workout, you set new PRs for 2:53 through 3:45. Click the link above and try creeping the interval shorter one second at a time and watch the average power. You’ll notice that the average power does not go up as you shorten the duration due to the spikes at either end of the workout. It trends downwards.
This is the 2:53 interval with an average power of 258. As you can see, you lose that spike on the left side of the chart, which makes the average power for the interval lower.
Does that clear things up a bit?