When driving across the country we've encountered places where the limit goes from 65 to 55 to 45 to 55 to 35 for no obvious or apparent reason.  A lot of towns do the same thing, as you drive down a street the limit is 45 then drops to 35 then back to 45.  No change in housing, no schools or business change, just the speed limits.

One place out west, may have been Nebraska or Wyoming we encountered a 35 mph speed limit about a mile from a sneeze and your through it town.  On the other side of town the limit jumped back to 70 very quickly.

Note, 100km/h is only 62 mph.  For simplicity  .6 x km/h so 30km/h is less than 20 mph.

There's a joke relating to this.