The Subscriber can have an incorrect/old APN and still work with new settings, cause of the packet core equipment can override existing APN as configured on the phone/modem with the a default one as configured by the telcos. 

They can do this to avoid inconveniencing subscribers when they make changes to the APN and also to reduce connection problems due to misconfigured APNs settings on the MS. 

This is an optional feature that can be implemented by the operators

regards

Patrick