The Power Duration Curve in Xert isn’t based entirely on actual watts you’ve ridden for X seconds/minutes.
A 5 sec burst is enough to validate Peak Power but there are circumstances where the algorithm is calculating your potential power curve points based upon performance under fatigue.
Here’s a good discussion regarding Peak Power and the concept of derived PDC in Xert.
Is PP a theoretical number? - General - Xert Community Forum (xertonline.com)