Can deleted device be reallocated in the Developer Portal?
Each year when you renew your program membership you can remove devices added in previous years.  So if you had 100 devices allocated, you could delete 5 and add 5 new devices.

As soon as you add a new device, you can no longer remove devices for the remainder of that year.

So what they are telling you is each year, after renewal, delete any devices you need to remove first. Once you add your first new device, you are only permitted to add devices until your next renewal up to a maximum of 100 devices.