Zwift FTP vs. other cycling apps

How do other users find Zwift’s estimate of FTP compared with those of other cycling apps such as Sufferfest? Based on various TT workouts, Zwift estimates my FTP as 272 W, but the last time I did Sufferfest’s ‘4DP’ power test, my FTP came out as 318 W. This seems like a big difference. Admittedly, that was some time ago, and I may now be somewhat less fit, but I don’t think so. On a similar topic, I just tried a flat TT using a standard Zwift TT bike with 50 mm deep section wheels (the only ones available in my ‘garage’) and I only averaged about 24.5 mph for a 15-mile session. On the road I’d expect to get an average of about 27.0 mph for that kind of distance with my normal TT setup. So do other people find Zwift to be, on average, somewhat harder than outdoors?

You can’t compare the 4DP test to a TT race they are very different.

There are many FTP test protocols all have strength and weakness.

Here is a good article that discusses why FTP and other similar metrics are just approximations.