The size specification is but to be treated with caution. The '65 reference was too narrow for my moderately bulging inflated 65cm Ball unfortunately much. I have therefore taken a 75er-regarding my '65 Ball. Although the fact is too large, but does the job.
A rough estimate should fit well the 75er-reference to a 75er-ball. The 65er reference should rather fit a 55mm ball. Schön would be a "real" 65er-respect.