It is fairly common for many movies and TV shows to have a relatively high rating when they first appear, only to have the rating go down over the following months.
People who are more eager/interested in the title (and therefore more likely to be inclined to give it a good rating) usually go see it as soon as it opens or airs. The initial rating therefore tends to be heavily skewed towards the high end of the scale. Over time, the title is seen by a wider audience of casual viewers and attracts votes from people who may not necessarily be fans of the genre or who have different expectations and may therefore have a less favorable opinion.
In the case of movies, this applies to major studio productions (where most of the box office is earned during the opening weekend) but also to low-budgeted/indie titles, for different reasons: with smaller movies, initial votes are usually cast by people who are either connected with the production (or their friends) or who had the chance to catch the movie at a festival or special screening, which tends to attract viewers who may be more favorably inclined towards it. Over time, the movie may get wider distribution or simply be seen by a more diverse audience, and their vote may be very different (and lower).
Similar factors apply to foreign-language films, which are first released in their country of production and may generate favorable ratings from their intended audience but may encounter a less positive reaction when the film travels outside its native country, due to language issues and cultural differences.
Finally, people may reassess their opinion of a film or show with subsequent viewings: a title that may have felt like the greatest in history when first viewed may reveal more flaws after a second viewing; a film or show that relied heavily on eye-popping special effects may have looked groundbreaking and impressive when it was first released but may fare badly after a relative short time as the visuals have become dated and more sophisticated productions have changed people's expectations and standards.
The main point to keep in mind is that user ratings are constantly evolving, and that ratings become more accurate as time goes by and as more and more votes are gathered.
See also the complete FAQ for IMDb ratings.