It's pretty straight forward. Users buying phones that cost $250 on average [1] will also spend less money on apps that users buying phones that cost on average closer to $700.
That being said, worldwide Apple only has 20% of the market now so while the average user is cheaper, there are quite a bit more users. That's probably why ad-based monetization also works a lot better on Android (more users).
Even though Android is 80% of the market, the numbers are still shockingly tilted.
People who choose the cheap option really don't do much discretionary spending, people with iPhones actually make up the majority of revenue for many apps.
That being said, worldwide Apple only has 20% of the market now so while the average user is cheaper, there are quite a bit more users. That's probably why ad-based monetization also works a lot better on Android (more users).
[1] https://www.statista.com/statistics/951537/worldwide-average...