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keyword spam detection will end up DIFFERENT for specific keyword phrases (certain markets and website taxonomies.) So the stuffing threshold will be set by a statistical values that are seen across a large number of sites in any given niche.
Articles will see a very different threshold than point-of-sale pages.
My observation is from a blog on a ecommerce site which gets desired ranking for some keywords, but less for others.
Also, is it possible that the information articles are getting more inbound links, perhaps from better sources, than the money articles are getting?good point; but it appears that the IBL is the same. As mentioned earlier all phrases are treated equally in regards to content, material & layout, so they should both deserve the same inbound link rate.
Comparison, reviews and other buying features also help.