Steve, I have seen this on many occasions.
There are 3 mains reasons IMO as to why this happens:
1) Not enough deep links to your site (try getting links to category pages etc)
2) Over duplication of the template. (What I mean by this is that sites that have sidebars with tons of links, footers with 6 lines of text, identical meta on each page etc... will have a high % of content that is the same on each page. Often, trimming the rubbish from the template, sidebar, menus and footers can sort this, also pay close attention to your meta info.
3) Duplicate content (Are you usinf articles from elsewhere etc?)
Dean
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