How Could Findmyiphone Track My Phone Even Though The Sim Card Was Exchanged

We got our stolen iPhone 4 back, even though they had a new SIM card and different carrier. I got an email yesterday telling me that the phone had been located. I tracked it down using the FindMyIPhone App and we recovered the phone.

I’m thrilled we got it back, but confused as to how the app tracked it, since the phone had a new SIM card and carrier.