There's no reason to expect that this cache would contain the IP addresses of every computer on your network. Cache entries will expire after a certain length of time, so if your computer hasn't contacted another one in a while you won't see that other one in the cache. Sometimes the cache may also contain mappings for computers which have announced their address or otherwise contacted your computer. Because there's overhead in performing mapping, computers maintain a cache of recently mapped addresses.
TCP/IP stacks use ARP to map between an IP address (like 10.0.1.1) and hardware (usually MAC) address, like 00:11:22:AA:BB:CC.
