If you change the allocation method to static, you as the administrator cannot specify the actual IP address assigned to the Public IP address resource. Azure assigns the IP address from a pool of IP addresses in the Azure region where the resource is located.
- Chapter 4 Implement Virtual Networks
- from Exam Ref 70-533 Implementing Microsoft Azure Infrastructure Solutions
- Publisher: Microsoft Press
- Released: January 2018
That is correct. It has happened to me. Azure has pool of Public IPs per region that can't he assigned by the administrator. Azure chooses one from there.