Gotcha,
I'm off by about 8-10% across the board but for me that's tolerable. Outdoors you can do so much more to generate power; use the arms, swing my bike to get leverage, etc.
Actually if that 8-10% is accurate my outdoor FTP is stronger than its ever been (we'll see)
I had similar issues with the Fluid Trainer; I did the Zwift beta on one and hated it. Having Direct Drive is sooo much better.