LAN IP address assignments - DHCP VS Reserved VS Static

Newbie with the Helix router.  Why is it that if I want to assign a static IP to my servers and printers, the address needs to be in the DHCP pool?  Seems conter-intuitive...  In my personal case, I wanted to reserve 10.0.0.0/24 IPs

• .50-.99 for network equipment
• .100-.125 for servers
• .150-.175 for printers
• .200-.253 for the DHCP pool

So I modified the DHCP scope OK, but when I then selected one of my servers from the device list and tried to make it static at 10.0.0.100, I got an error saying that IP was not in the DHCP range.  Feature or bug?