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 is 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 list only includes theatrical feature films. Shorts, TV movies, miniseries and documentaries are not included.