Possible Duplicate:
Should we reduce rep bonus for upvotes on posts with a negative score?
A lot of users will up vote if they see a question or answer that has -1 but is not totally terrible. So you then have (-1 + 10) = 8 rep.
So an answer that should have got no rep, gets +8 rep, due to the helpful person that down votes it. (Thanks for the down-votes, they give me lots of rep!)
What if instead we said:
If (totalVotesForItem => 0)
repGained = totalVotesForItem * 10
Else
repGained = totalVotesForItem * -1
Then getting one down vote and one up vote will give 0 rep gain rather than 9 rep gain.