FortiAuthenticator firmware version doesn't directly relate to FortiGate and other products' firmware version, as you noticed. Integration between FortiAuthenticator and Fabric devices is somewhat limited, essentially boiling down to RADIUS/LDAP/FSSO.
The firmware convention of 6.2/6.4/7.0/7.2 for various products is used largely with products that participate in Security Fabric: FortiGate, FortiAnalyzer/Manager, EMS, etc.
As for the next major firmware version of FortiAuthenticator, I can't say whether that would be 6.6 or 7.0 or something else; for example, FortiAuthenticator jumped from 5.5 to 6.0 a few years back with a major overhaul of the GUI (to be more in line with GUI in other Fortinet products), and then continued on with 6.1/6.2/etc as new features were added.
That is to say, the FortiAuthenticator developers decide on how the product will evolve in the future, and what major firmware version is the correct one to signal how new versions relate to older ones, or possibly relate to other Fortinet products.
I hope that answers your question!
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++