That is because you are paying for the premium of signing up through ios; apple takes a cut so to make money they raise the rates. It’s extremely common signing up to any service through ios, that you will be charged higher than if you went to the website on your browser and sign up for it there. First link has link to Hank Green who does a great break down and investigation but provided many more examples.

theverge.com/…/subscribing-to-stuff-from-your-iph…

www.devicemag.com/youtube-premium-cost-iphone/

lifehacker.com/dont-sign-up-for-subscriptions-in-…

forbes.com/…/heres-why-your-apple-app-store-purch…

umatechnology.org/psa-subscriptions-cost-more-whe…