My guess it was elements either lost or subtly heard in the standard mono that somehow reintroduced themselves on the mono track included on the MGM DVDs, either by accident or design. If you want a somewhat closer examination I shared two youtube videos below of the police station massacre. First has the standard mono, second has the MGM mono. Both are mostly identical but you hear subtle differences like the sound of the emergency lights coming on when the powers cut.
Neither link worked for me.