The 640-864 DESGN Exam covers:
Describe the Methodology used to design a network
Describe network structure and modularity
Describe network architecture for the enterprise
Design Basic Enterprise Campus Networks
Design Enterprise Edge and Remote Network Modules
Design IP Addressing and Routing Protocols
Design network services
Network design is all about proper "division of labor" - you need to use the right gears to do the right things at the right place. You are seeing things from a broader planning perspective. The security exam, on the other hand, is more technical and is more concerned on the security aspect of networking.
Topics that have to be dealt with in the CCNA Security exam include:
Common Security Threats
Securing Cisco Routers
AAA
Threat Mitigation with ACLs
Secure Management and Reporting
Mitigating Layer 2 Attacks
IOS Firewall Feature Set
Cisco IOS IPS
Site-to-Site VPNs