Have you ever seen your site as a self-referral? This will help to explain why!
When you open the Hostname report (found under the Technology tab in the Audience section), it will show you each hostname, or domain, that visitors used to reach your site. For the exact interpretation, you can read Google’s definition.
But in easier terms, I think of the hostname as essentially a website where your Google Analytics code is running. For example, if you copy the code from one page to create a new page on a microsite. Another common error is not to redirect visitors from the domain.com version of your site to the www.domain.com version of your site. When this happens, movement from one domain to other will result in a self-referral situation.
In order to fix this problem, you’ll first need to redirect the non-www version of your site to the www version (if that’s one of the hostnames that appears). Then you’ll need to remove the Google Analytics code from each of the other hostnames that appear in the Hostnames report. Some, you won’t be able to do simply because some pages are cached.
LunaMetrics offers a great rundown of some possibilities why additional hostnames appear in your report. However, the long and short of it is, the number of hostnames that appear in this list that aren’t your website will probably match up to the number of self-referrals on your site.
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One more reason 301 redirects are so important for SEO! You want the most accurate data possible, which means making sure all the entrance paths to your site are doing what they are supposed to be doing.