It means that you made mistakes in your calculation. The Cs value from Eq. 12.8-3 will never be greater than that from Eq. 12.8-2
If you take a look at the design spectral curve in Chapter 11, you will understand why that
Cs = Sds/(R/I) (12.8-2), when T <= Ts = Sd1/Sds governs
Cs =...