Well, simple logic dictates that something must be declaring the number of TM/HMs. They seem to be stored in the same list, judging by the bleed over. Even so, the combined number doesn't add up to the maximum number of variables (forgive me for stating the obvious, I'm building a foundation for my hypothesis). It would seem from your test that there are two missing factors. One must define the highest numbered object that is a "TM", this being why anything beyond that value is automatically classed as a "HM". I think it likely that the other missing factor must relate to which badge is needed for which HM move to be taught in some way. Since you can only have eight badges, any "extra" HM would have no "badge check" to activate it. I don't know if that helps, and it's only a theory until tested, but it might give you an idea of what to be looking for.