Cannot breed Llama with more than 12 inventory slots

I have bred approximately 200 Llamas from a stock of about 10 naturally generated animals I found in my world. Currently I have a few dozen that have a 12 slot inventory capacity. From these I have bred 130 offspring, and never once have I seen a Llama with more than 12 inventory slot capability.

Is this intended behavior, is this a possible bug, or is there just an incredibly low probability of increasing the statistic from the parents level?

Please let me know if I can provide any further data, or perform any specific test to help narrow down the issue.