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But all this is probably an attempt to find a pattern where there is none, and the date could be just one of those Google "features", like the link: command, that is really there to keep us guessing.
To get regular fresh tags, a pr4 min (home page) and very frequent updates of both content and more pages helps.
Fresh tags do not help in the serps, you can find fresh tags way down on page 70..it may appear fresh tags help because you are more likely to see them page 1 or 2 ...nobody goes past page 3 do they?