# Molar specific heat constant for polyatomic gases?

My copy of "Resnick and Halliday" states that

"$Cv$=$(f/2)R$ which agrees with the experimental evidence for diatomic gases($f$=5), but it is too low for polyatomic gases($f=6$).

Exactly what do they mean by the statement that I have highlighted in bold? What's too low? $Cv$? How come? Doesn't $Cv$ vary directly with $f$?

It means that equation underestimates the experimental $C_v$ value for polyatomic gases. Indeed, the experimental $C_v$ value is about 15% larger than what is given by that equation (source).