The nonlinear microwave properties of ferroelectric SrTiO3 thin films are characterized via complex analysis of the intermodulation distortion (IMD) signals up to high microwave power. The measurements reveal an unusual dependence of the IMD signals on the input power, which indicates the presence of two different nonlinear properties being responsible for the generation of the intermodulation. IMD measurements on tuned ferroelectric films and simultaneous conductivity measurements reveal the two different properties to be the nonlinear permittivity at low rf power and a finite nonlinear conductivity at high rf power levels, respectively. The IMD signal strength can not be explained in terms of the classical description based on a Taylor expansion of the nonlinear parameter. In contrast, simulations of the frequency spectra using more appropriate descriptions of two nonlinear parameters yield an excellent agreement between theory and experiment, and, thus, demonstrate, that the IMD experiments together with the simulation might provide further insight into the mechanism of nonlinear behavior of these material.