Spider seems to be linked to some form of head wobble. The head wobble, as I understand it, is on a spectrum from non-noticeable to quite the wobble. If the homozygous form the spider gene is lethal then you wouldn't see that portion of the punnet square show up at all. It would appear that you'd get 66% spiders and 33% normals statistically. In reality you'd be getting 50% spiders, 25% normals, and 25% (super) spiders that never made it to the egg stage. Being a homozygous (super) spider would mean that the embryo is non-viable and would stop in development early. If a spiderXspider cross was done then all the spiders would be het spider (visual) and the homozygous (super) spiders wouldn't have made it that far.
A a
A AA Aa
a Aa aa

Keep in mind I'm far from a ball python expert but I do study genetics. I'd love to study lethal/wonky BP genes in depth to fully understand what the root causes for some of the issues are.