It's certainly true that weaker events are not as robust in delivering a mild winter to North America.  One thing to be cautious about, however, is that the colder years were generally earlier during the period of record. Almost all of the later years were warm out West, hence the tilt toward above normal temperatures in the region of the country.  The East has been more of a coin flip, so the current outlook of equal chances fits with recent historical weak El Ninos.