Fluctuations of the proton number distribution in central Au-Au collisions have been measured by the STAR Collaboration in a beam energy scan at the Relativistic Heavy Ion Collider (RHIC). The motivation is a search for evidence of a critical point in the equation of state. It was found that the skewness and kurtosis display an interesting energy dependence. We compare these measurements to an equation of state which smoothly interpolates between an excluded volume hadron resonance gas at low energy density to a perturbative plasma of quarks and gluons at high energy density. This crossover equation of state agrees very well with the lattice QCD equation of state. The crossover equation of state can reproduce the data if the fluctuations are frozen at a temperature significantly lower than the average chemical freeze-out.