Stagecoach East Scotland's 87 was formerly an example. The service operated between Dunfermline and North Queensferry via Duloch Park and Dalgety Bay, a long route which took 1h15mins to complete. As there was little desire to use more than two vehicles on what was (and still is) a council subsidised route, the 87 operated at the odd frequency of every 1h20mins - 1h15 to complete the route with five minutes of layover time.
However on Sundays, while it still operated, it ran a much shorter section of the full route, only running between Dunfermline and Duloch Park. As this section only took 26 minutes to complete in one direction, an hourly frequency operated with one vehicle. Hence, the frequency on Sundays was hourly, with the route operating every hour and 20 minutes on all other days.
The 87 is now operated by Bay Travel and a number of adjustments, section swaps, and Stagecoach operating the Sunday service as part of a different numbered route (service 84) means this setup is no longer the case and the specific 84/87 corridor between Dunfermline and Duloch Park receives an hourly frequency all week.