http://msdn.microsoft.com/en-us/library/office/jj687488.aspx
I had the same issue, it turned out that Search Results web part was incorrectly trimming results that were considered duplicates.
I fixed this by setting TrimDuplicates to false. To do this:
- Export the web part,
- change TrimDuplicates to false in the DataProviderJSON property, and
- add the web part back to your page.
Hope that helps Christopher Scolt