As indicated on the IMDb Top 250 Movies and IMDb Top 250 TV Shows pages, only votes from regular IMDb voters are considered when creating the top 250 out of the full voting database. This explains any difference between the vote averages reported in the top 250 lists and those on the individual title pages. This also explains why movies or shows you might think from their averages ought to appear on the list yet do not actually appear there.
To maintain the effectiveness of the top 250 lists, we deliberately do not disclose the criteria used for a person to be counted as a regular voter.
Please be aware that the Top 250 Movies only includes theatrical features. Shorts, TV movies, miniseries and documentaries are not included.