A lot of stations feature special "package" rates in the fourth quarter, designed to run into the lowest billing periods of January and February. As a result, "frequency" of the spots, often at the lower prices of the package, are more frequent in November & December, and less frequent -- but still accountable for billing in January. Very common practice. Some stations will bill "equally" in those three months, no matter when the spots are aired, though suggesting that advertisers leave spots to run in January, when spot inventory is traditonally at its lightest.
For those no participating in the "holiday special" packages, "normal" rates still apply, unless stations are giving spots away off the rate card or for long term contracts.