If you're using "selection bias" to refer to the fact that these two groups are selecting movies using different, biased methods, doesn't that imply that there's some way of sampling that would not be biased? But no matter what you do, an average critic and an average moviegoer are likely to disagree on many films. I would expect the explanations behind this to be more of a function of differences between the groups, which explains 1. why critics will occasionally enjoy a film more than the general population, and 2. why this disagreement can drift over time. Both of which are not accounted for by your analysis. I'm definitely not saying you're wrong - quite the opposite, just that there are other interesting things to discover here.