I only have 13 ball pythons but out of that 13 I have two females who are really slow growers. One of them has not been a very good eater (and I'm working on that with her) and the other is just growing very slowly. All my other girls are absolute feeding machines!!
I think that there might be an average growth rate, but they also grow at their own rate. So, maybe you have a snake who might just not be ready to go next season. It happens....
Just a side note: 12 day feedings seem like a long time for a snake who weighs that much. Have you considered making the feedings every 7 days and sticking to that? It might help a little with weight gain