That could be the result of the paragraph formatting in the source document's table using Style definitions that have different specifications from those of Styles with the same name in the target document. Ideally, you'd use the same Style definitions in both documents - and not override those definitions with 'hard formatting' in either document. That way the problem wouldn't arise. That said, if you go to File|Options|Advanced>Cut, copy & paste, choosing the 'Keep source formatting' for both versions of 'Pasting between documents' should provide a workaround.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]