@davelr, sounds like you're applying custom durations to your zones? Rachio will recommend a duration based on the zone settings selected; curious if you're overriding these values?
I'm not sure if you've reviewed this support article yet, but it details the math behind how the curve is calculated and applied. My assumption without looking at the data is that your base duration (9 minutes) is the root cause for the 15 minute max durations.
Remember to factor the intervals of the watering into your math. Your controller is currently on Standby so I cannot review the schedules, but happy to check the intervals if you're okay with me taking the controller off Standby.