One phone, different access networks
On home Wi-Fi, traffic normally passes through a home router and a broadband provider. On mobile data, it enters the mobile operator's network. Different providers and connection paths can assign different addresses or present different exits to internet services. A change in address therefore reflects a networking arrangement, not a new personal identity. Even the same phone can communicate through different networks over time.
Local addressing and internet exits
A Wi-Fi connection may give the phone a private IPv4 address while a router translates several devices' connections to one public address. A mobile operator may also share public IPv4 space among subscribers. Consequently, an address shown in a phone's network settings is not necessarily the address a remote service observes. An exit address alone cannot reveal the number of phones or people sharing it.
Why the address sometimes stays the same
Different access paths can still lead through the same intermediary exit. An application may also retain an existing connection while a new path becomes available. Connection policies, proxies and VPNs can influence which route is used. A network icon by itself does not establish that every request has switched. Interpret observations after the desired connection is active and a new connection has been made, rather than relying on one unchanged value.
Address changes are not location measurements
A mobile operator's exit may be located in another city, and location databases can describe that exit rather than the phone. Switching protocols or providers can also change the reported region. IP-based location and device positioning are different sources of information. A new address does not prove that the phone moved, and an unchanged address does not prove that it stayed still. Consider the network and address family instead of using an IP as a permanent device identifier.
What about devices using a phone hotspot?
A laptop using a phone hotspot often shares an IPv4 exit with the phone through mobile NAT. IPv6, proxies and routing policies may instead produce different addresses. A shared public IP does not identify a single device, and one differing result does not establish that two devices are using different hotspots.