FDTD for patch antennas

ref the paper Numerical Modeling of Microstrip Circuits and Antennas, D. M. Sheen, Cambridge, MA, Massachusetts Institute of Technology, 1991.
The patch work 7.5GHz.
But in Sullivan's FDTD book,as a example for 3D type.But in his book,the result is 6GHz,is it the result error?(in CH5)