The system is set to treat any thread that hasn't had a new comment in 2 weeks or more as if you read it. This is for performance reasons. If it was marking threads as read had comments less than 2 weeks old that is a glitch :( Are you sure you were logged in? The only other reason they would show as read is if you weren't logged in, as it treats guests as if they have read everything.
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Guides By RestrictNine, A.K.A. Taggrin