It’s because the mobile operators differ in how they handle IP address allocation, Some operators have a fixed IP address for all of their users, others dynamically allocate IP addresses each time the device connects. But most mobiles, whether on 3G or Wi-Fi, will connect via a NAT-ed connection. This means many different handsets will appear to be coming from the same IP address, where the data connection is using a mobile network operator (MNO) gateway, the IP address will be the IP address of the MNO gateway, which will handle multiple user sessions. The handover from one mobile phone tower to the next can also result in IP changes plus dropping from 3G to 2G connection has an effect on the reliability of an IP address, and thus tracking these changes can be problematic.