An AP may join a different WLC than the one specified through option 43 if it has been primed to join another WLC. Priming is the process of pre-configuring an AP with specific WLC details, such as the IP address, so that when the AP boots up, it knows which controller to join. If an AP is primed to a specific WLC, it will ignore DHCP option 43 and attempt to join the primed WLC. References: Cisco Community, Cisco Community, Cisco Documentation