What its likely to be is your mobile provider, in order to save money is keeping an image cache that ignores the rules we set. So when you request the image instead of downloading it from our server, they serve a local copy which saves them having to do any actual work, and then they get to charge you for it anyway on your data allowance.

Its basically the LTE provider's version of 8AM Subway footlong sandwiches, you know they're the ones left over from the previous day and they're stale and dry, but they'll still sell them to you and the fresh ones will still be "cooking" until all the old ones are sold.

 

Once its been served your iPhone browser cache thinks the image is current for a period of time and keeps it. The image served by AT&T could have an expires header set 3 days in advance in order to minimise your data requests in which case even after you're on wifi that 3 day expires header will be respected by the browser.