The assume-good-faith answer to a speed limit dip like that is usually that the road has one or more intersections with some risk factor. Like limited visibility around a bend or hill, or a short merging/acceleration area, or prone to flooding in rain, or intersection with a bike trail. Of course, any particular spot like yours could be a police cash grab, but there are many places where something like that seems arbitrary but there is a real reason. Best to follow the signs, for conditions you might not know about.