It depends on how detailed you want to get on your zone settings.
The Fixed schedules will take advantage of using Smart Cycle (breaking up the durations to allow your lawn to soak up the water) and Weather Intelligence. It will run all your zones on the selected days.
The Flexible schedules requires you (to take full advantage) to input a little more data about your yard (Available Water, Root Depth, Area). It will create an optimize schedule based on these settings and will control when it waters based on your moisture levels. Flexible schedules will water only the zones that need watering according to the moisture levels shown on the Moisture Charts. So, maybe it is showing it in only running for 15 minutes because it is only running one zone. It is less predictable, but you will see the maximum possible water savings.
To take full advantage of the Rahio, I would suggest Flexible Daily. There are threads to help you determine the area of each of your zones, your Available Water, and other Advanced Setting features. There are also plenty in the community who can help you with these settings as well.