Why do web sites show my iPhone using different IP addresses for HTTP and HTTPS (cellular only)?

I am just going to take a guess here. Your telephone data carrier may have an optimizing or caching proxy for content whose IP address appears in your JSON result. As the proxy has no visibility into encrypted HTTPS packets, it cannot proxy the content, so it may be routing directly with your public (routable) IP address.

If this is the case, your phone has one IP address but the carrier's routing shows different origin IP addresses at ipof.in.

Your HTTP requests are proxied, hence the ipof.in service sees different IP addresses.

I believe this answer explains it well, but another possibility (regardless of cellular/WiFi connection) is that your browser uses a Data Saver (Chrome^*) or Turbo Mode (Opera) option. Both intended to compress the data for mobile devices.

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^* Chrome feature seems not available anymore for iOS. It could be turned off to get consistent results. Visual instructions for old Chrome (Settings -> Advanced -> Bandwidth -> Data Saver and switch the option to Off) are here starting from slide 5.