All the gametes a female makes come from one cell initially. So every gamete she has could potentially have the gene depending on how the genes sort out during division of the cells to produce eggs. She could keep throwing strangers for a long time, if the mutation started deep down in the stem of that cell line.

Codon repeats are pretty common in genetic diseases. Huntington's chorea would be one of them. Normal parents start having afflicted children, because the gene stutter gets worse. http://wwwchem.csustan.edu/chem4400/SJBR/lisa.pdf

Keep in mind these are guesses on my part. I have a background in biology, but have never studied the genetics of ball pythons or this trait. These are possibilities, but it could be something else entirely.

David