Flex schedules view each zone as a bucket with an amount to fill up (soil moisture level).
During the winter there still is evapotranspiration (taking water out of the bucket) happening, even if it is almost non-existent.
Funny story. Had a customer concerned that his flex schedule was disabled and it ran the other day. Turns out, the schedule was enabled but hadn't run for about 30 days because his daily ET values were about .03in/day plus precipitation events. Assuming you have a bucket that takes about .7in to fill up, you can see where the confusion would be.
Our software does not currently shut off for your winter months, but something we would like to incorporate someday.
I believe most people have a normal shutdown process where the blow out their sprinklers and put the Iro in sleep mode.
Hope this helps. We are working on even simpler, more intelligent, more feature rich schedules. Not committing to any dates, but stay tuned.