Thanks for sending in a ticket to support on this one, @edbianc. It allowed me to take a closer look at your account settings in order to advise. I'll reply to that ticket with greater detail, but you asked that someone comment here. You have a somewhat unique use case here. So, for the benefit of the community at large...
Seems like there are two main issues at play...
-- First concern | why is Flex watering [almost] daily?
Modawk hit on what I think is the crux of this, when he spotted the soil type - Sand.
After examining your settings, it looks like you have a lot of them "dialed in" (i.e. - less default values than we often see)
So, I'm pretty sure you know your soil. Just in case you haven't, or for the benefit of other readers, some recommended reading -- What type of soil do I have?
From that article:
Sandy soils are the opposite of clay and generally drain too fast, and so are unable to hold onto any nutrients long enough for a plant to use them. Organic matter helps to hold onto water and nutrients, and as with clay, it’s almost impossible to add too much
This is a valid conclusion here, and it sounds like it matches what we're seeing. The zone in question (zone 5) is paired with a seemingly-identical zone 4, in a Flex Daily schedule. I'll leave a few additional notes on this in your support ticket, but...
This brings me to your primary question.
Second concern | Why did Flex water on a day with so much rainfall?
Linn's initial guess (below) is pretty dead-on here.
It appears Flex did its check that day at 3:16 AM, for a schedule set to start at 4AM. I'm not sure what it saw exactly at that point, but... it decided your warm season grass needed water.
In your first post, you mentioned that on that day there was a ...
However, I'm not seeing that reflected by your weather station, which shows 0.00 precip until 6am, then a static 1.27in daily total for 10hrs. At about 4pm, a final shower seems to have pushed it to the 1.65in total you see for the day. See: http://www.pwsweather.com/obs/archive/2016/07/21/KSCAIKEN23.html
Of course, the Rachio can only go off the data it's given. It could be that your selected PWS, at almost 3 miles away, didn't record the same rain you (or your grass) saw.
I hope this helps clear things up a bit. Flex has a lot of moving parts, but if you'd like to "game" it a bit there's good info in this support article on what goes into each "lever" : Flex Schedule Tips