I've noticed that some of my skins are getting hits on Google for things other than the actual name of the skin. Here is something to think about.

Skin tags. When uploading a skin, take the time to write some keywords in the skin tags section. Write anything that may pertain to your skin. In my Assimilation WB, Iv'e written Borg, star, and trek. That way if someone does a Google search for Trek window blind, they are more likely to stumble across mine!

Comments and description. There are some words that people type into search engines more often than others. I've seen my skins come up on search engines, because of a user comment or my initial description of the theme. If you write a Description like this: "This skin is bloody sexy. Even girls will like it!" you have allready put in at least 3 keywords that are sure to get hits on Google!

This is my theory anyway, but take a look at your gallery and check out the referrals to see why your theme get hits outside of WinCustomize.