I’m NOT from the VirtuGO team but have seen a difference for my FTP values too.
FTP (meassured with Ramp tests and Normalized Power of 1 hour online races as well as “autocalculated” by Golden Cheetah) in the range of 255-275.
VirtuGO FTP test 305
But … I was riding above 305 Watt for some time which seems to be different to your described behavior.
Any test that runs less than 1 hour will use some calculations to get from the meassured results an 1 hour value that will work for many but not for all.
In my VirtuGO test I remember that I intensivated my performance at the end which might have gotten me too high results.
For Ramp tests (e.g. You increase the power each minute by 25 watt until can no longer finish a one minute period and take the average of the last finished 5 minutes) a big question is what factor shall be applied to calculate out of the 5 minutes a 1 hour value. For me a factor of 80% seems to be quite accurate (I’m a hobby rider).
A good verification will be to load your test data into some analysis software.
Open source software Golden Cheetah is one excellent example.
This will give you a second opinion without the need to rerun your test
P.S. The generated VirtuGO trainings (based on 305 W) worked o.k. for me.