Forgive me, but what has caching speed got to do with links pointing to a www. url as opposed to the url without www?
It's most probably happening many times, I think there no need to worry or it's not a major issue, it's showing as back links where you have www or without www to your website. By the way you can solve the canonical issue using 301 redirect permanently, also mark one more thing there may be index.html or index.php means yourdomain.com/index.html or index.php
I things its not a problem. In general when we change the type of wording used to search somethings on SE surely it will show various results. Its just like that only. No need to worry.