My indoor Zwift ramp tests are always significantly lower than outdoor 20-minute tests. In general I find indoor rides harder at the same power. I do very little indoor riding as I hate it with a passion.

Also, I rarely do FTP tests. I think they have their place as a benchmark for creating training targets, which is important. But, if you’re not following a training plan, I find it close to useless as a gauge of general bike fitness. Or rather, I find my average power over a given route to be a better gauge.