It could be that he feels more secure on the cool side. Are the hides the same? If you have him in a glass tank, maybe is the cool side pushed up against the wall to where there's no activity (people walking past, looking in, movement, etc.) around that side. Ball pythons really have a need to feel secure and covered, so maybe, even if he does get a little cool, he feels more protected and secure on one side versus the other.
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