We've had this happen. We also have one girl who has never laid a 2 egg clutch. I've never researched enough into how the reproductive system works to answer why/how this happens.
I had this happen to me as well. My mack snow enigma female laid 4 clutches of 2 then all the sudden laid 1 egg and then laid another clutch of 2. Don't know why??? but the 1 egg she laid hatched today so I am not complaining....