It's really hard to compare the two, almost apples and oranges. The fixed schedule runtime is determined with real time weather data, which could essentially change from week-to-week, and then watering budgeting is applied. Eventually I want to move away from that since it is so dynamic and look at something a little more traditional, but maybe at the end of the season.
Flex schedules will have the same watering duration throughout the year, the thing that changes is the frequency. For example, my yard isn't scheduled to run for quite some time because of all the rain we are getting. But, we will water the same amount in May as in July, just much different frequencies.
Hope that helps.