Bug: new articles coming in despite "manual update"


I have all my feeds set to "manual update".
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However, when I check out my unread feeds in FR, after a while, some feeds re-appear with new unread articles in them (before, they contained no new article and were hidden with the "Hide read feed" option).
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Why do I get new articles coming in, although my setting is "Manual update" for all feeds?
Could you explain how "manual update" is supposed to work?
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