It could be the link types selected in the options tab. For example, the buddypress engine can make contextual articles, contextual profiles and microblogs (might be a different one).
Anyway, you may push a link through filtering as a do follow article but if the project you push it to has microblogs enabled it will make a no follow to.
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Anyway, you may push a link through filtering as a do follow article but if the project you push it to has microblogs enabled it will make a no follow to.