Let me rephrase the question as two separate questions. In Windows DHCP:
Q: Does the Reservation have to be in the Scope? A: Yes it does. It has to fall within the subnet defined in the Scope as determined by the subnet mask configured for your Address Pool.
Q: Does the Reservation have to be in the Address Pool defined in the Scope? A: No it doesn't.
You're mixing up terms here. A Scope is more than just the Address Pool defined for the Scope. A Scope also includes Reservations and Scope Options. Think of the Scope as a container. You define the objects in the Scope such as the Address Pool and the Scope Options.
If I have a Scope with an Address Pool defined as 192.168.1.100 to 192.168.1.200 with a subnet mask of 255.255.255.0 then I can create any Reservation that falls withing the same /24 subnet, such as 192.168.1.25.
What I can't do is to create a Reservation that falls within an Address range not defined in my Scope as determined by the subnet mask defined in the Scope, such as 192.168.2.25.