In brief, it is because internet connectivity on most cellular networks goes through single gateways and devices are effectively "tunnelling" to those gateways. As long as the tunnel stays up and the mobile device is still identifiable on the RAN then the IP address assigned from the internet gateway doesn't need to change.
What I found especially interesting is how it keeps track of the device, especially when transferring from say a LTE capable station to a station that only supports 3G.
