Women's representation in physics lags behind most other STEM disciplines. Currently, women make up about 13% of faculty members in all physics degree-granting departments, and there are physics departments with no women faculty members at all. These two data points are often cited as evidence of a lack of equity for women. In this article, the authors demonstrate that two factors affect the representation of women in a single physics department: (1) the number of faculty members in the single department; and (2) the overall proportion of women among all current physics faculty. The authors begin by presenting the current situation of women's representation in physics departments. Next, they present the results of simulations that calculate the expected number of departments that have no women faculty members. Finally, they compare these expected proportions to the actual proportions of departments with no women. They also include an analysis using hypothetical values for the proportion of women among the physics faculty to examine what might happen as the representation of women increases.