The statement “there’s a 40% chance of rain at any given point at any given time in the forecast area/period” is an average over both area and time.
Many different actual distributions of rain could result in that average, including a 100% chance of it raining 100% of the time in 40% of the are or a 40% chance of it raining in 100% of the time in 100% of the area, and a 100% chance of it raining 40% of the time in 100% of the area. Real distributions are typically messier than that.