Winter is the Best Time to Sell?

It has always been the assumption that Spring/Summer is the best time to put your house on the market. When looking quickly at the statistics for the most recent 12 months it's easy to make that assumption but I'm always wondering about the probabilites so I took a different look at them. December demonstrates the higher probability of selling than any other time of the year with October and November showing as high as June and August. Ironically, the Spring was the worst time to sell. I"m curious to know if this has always been the case or if like the weather, it's not as predictible as it once was.