Is the xert FTP and 20min power correct?

Hello. First i am sry for my english.
The question is. IS the xert FTP and 20min power estimating correct ?
Today i made 2x20 min Workout. The first session i started after 1h30 min into the workout. I made the first session 350w, the last 7 min was around 400. the second 20-25min session was easier around 320w. The hour power with the recovery between was 306.
I uploaded the workout and the xert FTP was 345. is this number correct ? or how the software estimate the numbers ? For example after last year race the Xert told that i have an FTP around 370-375 watts. I say these numbers are too high for my weight…

Thank you for the answers. Cheers.