There is no way they have 1 admiral tier, they are not even part of the balance of power.
There's also the thing that the Marines and Gorosei don't always look eye to eye (like their small quarrel with Akainu) so it makes sense for them to have their own top tier(s) working directly under them in case they are needed.
I think you're underestimating the CP0 here. They're being hyped especially for the final war imo.