I was looking in GWMT recently and noticed 400 more URL's than posts published have been indexed. I guess its because of image attachment
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Genesis Basic Custom Attachment Page File
You can use this attachment page file to create you own custom attachment page for all attachments or just your image attachments. Update
5 Ways To Change Default Image Attachment Links
Every time you upload an image, WordPress generates another post for that media file which is loaded using the attachment.php template.