It's confusing to explain
First, when making the zone you set your Allowed Depletion percentage, like we mentioned before (how dry you want your soil to be before you water again). This creates the bottom line (Allowed Depletion) and top line (Field Capacity). These lines are somewhat theoretical and have no true values associated with them, only relative percentages. Now, image that between those two lines is where your moisture balance hangs out. The range between the 2 lines is 0 to 100%. The 39% you see on may 22nd is where your moisture balance was for that day between the 2 lines. That 39% has nothing to do with your Allowed Depletion percentage you set in your settings.
I bet at the time the system checked the day's forecast, the Previous Moisture Balance minus the forecasted ET plus forecasted Precip put you below your Allowed Depletion (AD). After the actual events took place, it's clear that you would not have gone below your AD if the system didn't water.
Maybe they do. But is it useful data to have presented to us on a continual basis? I think once you have the concept of what's going on with the Flex Daily, it all makes sense and the forecasted information of the past isn't needed.