I have the Xert - What’s My FTP app on my Garmin and am a Xert subscriber. At the end of a ride my FTP reading on the What’s My FTP app will always read about 5 - 10 watts higher the Threshold Power reading on the Xert website. Why is there always a difference?
FIrstly, the What’s My FTP? is stripped down and makes some broad assumptions about your signature characteristics. Having said that, if you don’t set a high PP for the app, it will likely overestimate. Be sure to do a full maximal 5s sprint with the app, as per the instructions, to get best results.