During my time looking at the data, I'm starting to realize that many of the movies only have the top three billed actors listed, even if there are many other major characters credited for that movie. I believe this is because on the summary page, IMDb lists the top three actors (see below), and you have to click to see the rest, meaning some only enter the three they see there, and then move on.

main summary page

after clicking "see full cast and crew"
This is an issue because this means our data is inaccurate, and we need a better way to fact-check incoming pull requests.