I posted a bit of a complaint about the new search function and was told they are still tweaking it, and it's not quite working yet.
In other forums and search engines, the % score is a measure of how closely the post meets the search criteria. For example if you searched for posts with several words in them, and the search retrieved a post with most but not all the words, it might get a score of 75%. If it contained all the words, it would be 100%. However, for now it looks like the search function is still a bit wonky, so probably just ignore the % score.