This has been discussed many times, but nobody knows how it is calculated. But here are some facts:

• Blogs seem to be worth more than comments.
• One blog/comment with high magnitude seems to be worth more than many with small.
• Recent content is worth more than old.

Also, there are some accounts with negative contribution but seemingly not enough content to get that. They did not get their negative contribution randomly or anything like that, they have low-rated comments or blogs that have been deleted or hidden. Examples: CyanGuy, finneganpierson.

People can delete their blogs. But votes received on the blogs till that point are already added in contribution. Moreover, for obvious reasons people are more likely to delete blogs with negative votes.

If you edit or delete your blogs or comments, the contribution might get lower.

The few initial votes for a comment or blog are not shown. But they are included in contribution. Maybe this is the reason. The reason for not showing is most people follow the initial trend of downvote or upvote, leading to ...

• It depends on rates people who vote up or down .
• Voting in the main contribution is not counted if the voter is less than a specialist.
  Therefore, the number of those who voted and their votes were not counted may be greater than those whose votes were calculated.