There are substantially more movies recently, eyeballing it it looks like about 150 thousand movies were made in the most recent 12 years of the chart, compared to about 350 thousand made in previous years total. That would imply that recent movies are underrepresented very slightly. Removing short films might smooth that slightly, but actually these numbers are about right.
Upvoted for doing the research. Woo null hypothesis!
IMDB top 250 list is dominated by old movies, which conflicts with my perception (shared by majority of people as far as I can tell) that new movies are far better than old movies (comparing either top with top or average with average).
I have a simple theory why IMDB is wrong: