Ultimate 500 modems

Just confused at why modems rated higher than 500mbps are stuck at the Ultimate Classic tier, which to my understanding is 300mbps and not even offered anymore (grandfathered plan)

- If the modem is rated for 680mbps, what's the technical limitation as to why it can't support 500mbps?

- The current tiers go from 150 mbps service to 500 mbps service.  If you don't even offer Ultimate Classic service anymore, why is it listed on the Cox compatible modem page https://www.cox.com/residential/support/cox-certified-cable-modems.html