It's a stupid law that has been on the books in Japan for almost a century now. No one wants to be the politician that campaigned on making porn better, so even though there's pretty much universal agreement that it's stupid and pointless, no one is going to risk their career to repeal it. At the same time, there's a whole cottage industry around verification of compliance, and those businesses don't want to lose their cash flow, so they fight any effort to repeal it, etc.
Anyway, point is, the reason it feels stupid and arbitrary is because it is. Studios that fail to comply with the law are hit with hefty fines or being forced to close, so they go as close to the line to comply with the law without getting in trouble.