The iPad store didn't break away from the iOS store until 2019. The original app store came out in 2008, eleven years earlier. While you have as certain point, the division is meant to highlight apps which treat the iPad as a mini laptop, with a keyboard and pencil, vs just a bigger phone. If you've ever coded a website to resize, that's child's play. to actually get an app working properly across both sizes, you've got to put in significant work for each. For developers that don't want to put in that work *, listing in one or the other is fine, and possible Uber the current regieme.
* the developer for papers please's blog about the mobile port is indicative of the work that goes into supporting other screen sizes. Asking developers to have infinite time and motivation is just not realistic. also note that it took a few years for the mobile port to be released.