iPhone has WiFI turned off, but router is asking ARP to IP
Turning WiFi off on the iPhone means the device shuts down the radio - it means it doesn’t establish a network connection and route traffic. 

We would need to see what “the IP address” is and know details about your network to dig deeper, but here are how the system is designed. WiFi can work in airplane mode and turning off wifi is more like releasing DHCP reservation than not broadcasting. 


https://support.apple.com/en-us/HT204234
https://support.apple.com/en-us/HT208086


It also matters which iPhone and which iOS you have.