That improvement is definitely plausible. I think that on any given day, one’s performance on a ramp test could vary +/- 10 watts FTP (maybe more) – especially if it’s your first (or second) ramp test. Imagine the difference between feeling an off-day vs. an on-day. In my experience, I began using TrainerRoad last April (2020), and from April 5 to May 28 (around ~7 weeks), boosted my FTP by +33 wats – 211 to 244 – about +16%. It was my first time training on a smart trainer and doing so consistently, though I’d been a long-time 3-season cyclist (not much in winter), year-round runner/swimmer, and was a D1 rower, so I did have some long term endurance built up. It seems fairly likely that the largest % gains would occur in the first weeks of consistent training, though this may vary more widely that I envision.