@Linn Is correct. The flex daily schedules do not actually "skip" schedules, as much as push them back until they are needed. What this effectively means is that if it does rain, or if it is nice and cloudy, the Rachio will go ahead and push the next schedule back until the moisture levels drop far enough to justify a schedule to water.
As for when this happens, the Rachio checks the current weather, the past 24 hours, and the future 24 hour forecast in order to determine if a watering is needed on that day. If it rained enough yesterday/raining now/will rain enough tomorrow that there is enough moisture to push back a schedule, the system will do so.