Like I totally believe you, I'm actually just interested.
This distinction isn't arbitrary at all. They're effectively different markets. iPhone owners make more money and spend more money than Android users.
There have been a number of surveys that have shown this, here's one:
https://www.prnewswire.com/news-releases/iphone-users-spend-...
And an older one, but from a bigger source:
https://www.businessinsider.com/the-median-income-for-iphone...
Same with Apple's 30% fee - it's unavoidable on their platform so it acts as a private sales tax on their platform. Some developers might choose to essentially subsidize their apple users by charging them the same price on Apple and on tax-free platforms like the web. That may be out of fear for Apple or out of cultural inertia or whatever, but it doesn't make sense, they're the ones leaving money on the table, not the ones who charge people taking into account the cost of doing business with those people on their preferred platform. They could get more profit selling their app for cheaper when not subject to the 30% platform tax, if they think that their pricing with that tax is optimal.