It's because iTunes has a fee (from what I understand) from IAP, so in order to make Android profit per platinum = iOS profit per platinum, they have to charge a bit more.
That's one of the perks of android I guess, free market and credit card payments so there's no middle-man. Unfortunately for iOS, iTunes (Apple) is the middle-man through the App Store.
If iTunes doesn't get a cut from IAP, then I have no idea
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