Zydeco lays a jumbo egg

Well, if I am interpreting the United States Standards, Grades, and Weight Classes for Shell Eggs correctly, 71 grams is the official weight of a "Jumbo" egg (the largest classification.) Well, today our very own Zydeco (far left in this photo) laid a 74 ounce monster which dwarfs any other egg any of the three have laid yet. Out of curiosity, I weighed her last two and they were 52 grams each (I think they lose a tiny bit of weight each day?). The others' eggs ranged as low as 46 grams (barely half-way from "Small" to "Medium").
We've given them a sudden feast of scraps, but all three of them have had equal access to that. What would cause just one of their eggs to be so big all of a sudden?