I would not dismiss it as seasonality too quickly, though. A relatively marketable group taking around 57 days while weaker stock accumulates is exactly what a selective market could look like. The missing piece is whether completed volume held up or fell as inventory increased.
Also, what does 57 days measure: time until contract, closing, or removal from the market? Completed deals will naturally look older if the clock runs through closing. Without matching the definitions, the quick-moving listings and the price gap may not actually conflict.