Why is simple so hard? New Washington Flyer Coach to Dulles schedule is not intuitive.

Silver Line service started up this last weekend, and with it new Washington Flyer coach service to Dulles airport.  The new service runs from the Wiehle Avenue station direct to Dulles and back.  Fare is $5 each way.  (Previous service was from West Falls Church and cost $10 each way).

The new service is more frequent, too.  Here's a link to the schedule.  For weekdays buses run every 15 minutes from 6am to 9am; every 20 minutes until 3pm; every 15 minutes until 7pm, and then every 20 minutes until 10:40.  This schedule is relatively easy to remember without needing to carry it with you, because the service coincides with the top of the hour.  I.e. 6:00, 6:15, 6:30, etc..

However, the weekend schedule is not intuitive.  Buses run every 20 minutes all day from 7:45am to 10:45 pm.  But instead of running on the intuitive and easy to remember :00, :20, :40, they run on the non-intuitive :05, :25, :45. 
Why? 
What is the point of making the schedule difficult to remember for riders when it would have been so very simple to just start and end the service on weekends 5 minutes earlier?  I don't get it.