Casey even mentions the Nov-Nov chart period in his summary. He actually said "November 1975 through November 1976" in this countdown.
Then he means the end of November, because it includes all of November. The last two weeks of December typically have no published chart because Billboard takes the weeks off. So you're talking about two weeks missing from that chart, and those spins are counted in the next year's chart. It's a function of total spins, not weekly chart position, that determines the year end numbers.