I think that's purely because up until the Rose scene, which is the last one, we have no reason to believe otherwise. We don't have any context for any of them, apart from Luke's age, and Martha's new hair, so it's kind of straightforward to assume they take place, more or less, reasonably shortly (a few months?) after we last saw each of the characters in question.
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Date: 2010-01-02 12:48 pm (UTC)