It's not visible for the iOS user that they pay more without explicitly looking it up. And the difference is small.

Just add a transparent reporting about how your prices came to be and they will not let you do so.

Are the subscriptions portable between the web and mobile apps? Or only valid one place or the other?

It's account-based, a sub on one transfers transparently.

kind of? They have a special category for mobile subscriptions, and the sub system on mobile uses a token system. So while they all do the same thing, they are actually very diverse in how they are operated. A sub on a phone still gives you the benefits on any platform though, but it will continue to use the payment gateway for the life of the subscription

Well, Apple doesn't actually allow you to sell an app for $5. You can only choose $4.99 or $5.99.

I'm sure they're really hurting for that cent, you think?

Not at all. Just explaining why there’d be a price difference. Apple has a good reason for not allowing weird prices like $3.61 but also excludes prices like $5. They also used to require that subscriptions outside the App Store match In-App Purchase prices, but dropped that requirement.

Pricing $5.99 in-app and $5 outside is a good strategy. It’s a nudge to get users to buy outside the app and save 15%, but if not they’re effectively splitting the Apple tax.

Your second paragraph explains why you think there would be a price difference. That you can only choose 4.99 or 5.99 seems unrelated to your actual theory, and it doesn't make sense on its own.