I'll take the 83w. 90w minus 83w is 7w. 7w is nearly 8% of 90w, if we take 90w at 20min to be your max.

Therefore, it's entirely possible you could do an hour ride at 83w instead of the 15min at 90w.

92% of what you can do for 20-ish minutes is what many folks can do for an hour. Yes, that 7w could make that difference in time duration you can do something.

In the meantime, take 92% of your 90w figure and put it into the workout settings "ftp" since that's the best you got for roughly 20min. Try again, and see how workouts go.