IMDb publishes weighted vote averages rather than raw data averages. The simplest way to explain it is that although we accept and consider all votes received by users, not all votes have the same impact (or ‘weight’) on the final rating.
When unusual voting activity is detected, an alternate weighting calculation may be applied in order to preserve the reliability of our system. To ensure that our rating mechanism remains effective, we do not disclose the exact method used to generate the rating.
See also the complete FAQ for IMDb ratings.