I don’t know the exact formula, but I believe it totals the votes for all submissions you made that week, figures out the average total for all players, and then subtracts a score if your total is a certain number below the average score, or adds if your total is a certain number above the average score.

So, yes, you can have something accepted in and still go down if you had lots of other items that were sent to -5. Likewise, you can have nothing accepted and still go up if you submitted lots of good-but-not-good-enough items, or had say the second-highest forum idea.