I wondered about the same thing when I was using the A90 with the Phantom. In my setup two of the ridges are in contact with the Phantom headshell while the left rear ridge is completely outside the headshell area - see this pic.
The same situation existed with the Ortofon Jubilee in my setup.
In practice the A90 sounded great and I doubt contact with the third ridge would have altered the sound - at least with an arm having easy azimuth adjustment like the Phantom.
The same situation existed with the Ortofon Jubilee in my setup.
In practice the A90 sounded great and I doubt contact with the third ridge would have altered the sound - at least with an arm having easy azimuth adjustment like the Phantom.