If you could let me know what schedule type you are using, I can get more specific with troubleshooting, but I will give some general info around what I think may be happening.
If you are running a flex daily schedule, the duration of each individual zone watering will never change. The only thing that changes is what group of zones runs each day. There is a chance that more zones ran this day than normal.
If you are running a fixed schedule, I am guessing your schedule was seasonally shifted. This is a weather intelligence feature that adds/removes time from your schedule based on historical weather conditions. If you disable this, you should not see the time increase or decrease.
Like @ssindelman mentioned, Smart Cycle does not change duration.
I do agree that being able to set a "max" watering time would be a valuable feature, and will make sure to relay that to our team. In the meantime, we will make sure there are no more nasty letters!