ContentDownloadTime

Hi, Team,
Need Help, WPT giving more contentDownloadTime compared to the performance API, it is almost 10-90%,

Scenario:
We ran the WPT instance and executed our test, in the same machine we captured the performance timings using the performance requests, but the performance API entries are showing very less compared to WPT. But in our local browser, timigns are same almost. COuld you please explain why this happened this diff if we captured both details in same machine?