A couple thoughts...
It is talking about President, but counts years where those names appeared on the ballot as Vice President too.
It also ignores that a lot of other people have also been on the ballots over the years.
Also, some of this is natural. When a president has an 8 year run, the Vice President is a natural person to take a shot at the presidency, so it's perfectly normal to have 12 years where the same person is on the ballot as VP then P. It's natural for this to happen more than once in a row. Whether we've had this long of a streak or not, I don't know, but I suspect we might have.
This showerthought covers 44 years. The Bushs and Clintons double dipping is 12 years worth of ballots, but the other 32 years were natural. Hilary lost when she tried to double dip so I would argue the 8 years of Dubya are really the only anomaly in actual presidencies as far as "dynasties" are concerned.