Chapter 18 Question

In figure 18.29b on page 669 of the text it shows a model of the recifier output port with the constant and time-varying power souces separated. The time-varying source is negative and shown with a squared cosine function. Equation 18.93 is a calculation of the instantaneous output power which is a sum(in this case it is a difference due to the negative term) of the two power sources. In equation 18.93 the cosine function is not squared but in the model it is squared. What am I missing?