They could be using a network carrier which does not assign an unique IP address to each user but instead has only one external IP and uses Network Address Translation to allow multiple users to browse the web with that address. Such providers are quite rare in North America and Europe, but more common in other parts of the world.
There are countries where iPhones are sold exclusively with a plan by a single carrier. In that case all iPhone users from that country would have the same ISP and when that ISP uses NAT, also the same IP address.
In that case, a geoIP database would give you the location of their provider. These databases have very varying accuracy anyway. For my current IP address, all databases I tried return just the city I live in, which has a population of over a million.
Another explanation for multiple users sharing the same IP could be that they are using the same proxy server for anonymity. But having the same mobile phone and also using the same proxy service would be quite a coincidence.
bottom-line: There is a possibility that you are dealing with different people. However, it would be quite a coincidence. When you have other reason to believe that you are dealing with one user with multiple identities, it is quite likely to be the case.