Why does one marketplace use a longer window than the other?

A window is a trade-off: longer is statistically sounder and slower to notice change; shorter reacts faster and mistakes noise for signal.

A marketplace with a third of the traffic needs roughly three times as long to accumulate comparable evidence. Using one window for both would mean either acting on noise in the small one or acting too late in the large one.

More important than the length: the window should begin after your last significant change — a deal, a stock-out, a restructure, a prior optimisation.