A 6% score is negligible and means nothing. It probably means that you happened to put a thought together using some of the words that the site in question used (which can easily happen unless they are all in the same sequence and not a common phrase), or that you used a quote (even if properly quoted) found on that site.
Remember – the percentage matched is only to alert readers to a potential problem, not to confirm a problem. Most teachers and professors would not even bother looking at a 6% match, and I can’t imagine the UC admissions team will, either.