It's simply a weakness in the software of the harvesters interpretation of relative links.
It's an easy recognition of harvesting.
adding a deny for these types of link-misinterpretations is relatively easy.
g1smd
3:26 pm on Feb 25, 2011 (gmt 0)
Run Xenu LinkSleuth over you site to ensure that you haven't got some links pointing to the wrong place within your site.
I found a site that had a page, for example, at example.com/folder1/page1.html and it wanted to link out to another page at example.com/folder2/page2/html.
I would always use a root-relative link href value something like "/folder2/file2.html" beginning with a leading slash.
The site had tried to use "../folder2/file2.html" but had typed it as "../../folder2/file2.html" instead.
That URL is, technically, above the root. In reality the server still sent the right page, but Google wasn't crediting any of the links.