We had a similar issue with our system, thinking we had it set correctly prior to leaving in March and then receiving our April water/sewer/irrg.amenities bill @approximately $310.00 for the month, our estimated irrigation water usage was 37,000 gallons plus, so we had a friend of ours go to our house and turn it off completely and he would come back once a week and manually run it for us.
We now just manually run it when we are there, as we don't trust setting it while not there for fear of an outrageously high water usage/bill.
I understand what you are going through, thinking you have it set correct and it comes on and runs, then randomly comes back on again, it's not a good thing.
The irrigation people come out and give you a lesson on how to set it, but never give you information on what to do if things go haywire!!!
I see where many people recommend unplugging the whole system and removing the battery for approximately 10 minutes, I guess if that's the only way to clear the memory then I'd go for it.
