Ssh – Why does SSH add a known_host entry for an IP address

I have a host named nms.example.org. In my /etc/ssh/ssh_known_hosts I have an entry for the host with the RSA key. This entry, and all the other entries are managed by my configuration management system.

nms.example.org ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDZqfmVPs/XqTS...

Also I have an entry in my /etc/ssh/ssh_config for the specific host setting the host key alias. Which if I understand everything properly, this means that only the nms.example.org should matter.

Host nms.example.org nms.example nms
    HostKeyAlias nms.example.org
    HostName nms.example.org

Why then, when I connect from a client does ssh still seem to think it needs to add an key to my per-user known_hosts with the IP of the host?

$ ssh nms -v
OpenSSH_6.0p1 Debian-4+deb7u4, OpenSSL 1.0.1e 11 Feb 2013
debug1: Reading configuration data /home/zoredache/.ssh/config
debug1: /home/zoredache/.ssh/config line 61: Applying options for *
debug1: /home/zoredache/.ssh/config line 71: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 84: Applying options for nms
debug1: /etc/ssh/ssh_config line 363: Applying options for *
debug1: Connecting to nms.example.org [104.236.190.144] port 22.
debug1: Connection established.
debug1: identity file /home/zoredache/.ssh/zoredache-20140204.id_rsa type 1
...
debug1: Server host key: RSA 6b:5f:b6:e9:13:c3:b7:39:1e:ec:74:05:33:64:4d:5e
debug1: using hostkeyalias: nms.example.org
debug1: Host 'nms.example.org' is known and matches the RSA host key.
debug1: Found key in /etc/ssh/ssh_known_hosts:104
Warning: Permanently added the RSA host key for IP address '192.0.2.144' to the list of known hosts.
debug1: ssh_rsa_verify: signature correct
...

SSH knows my host is valid (See: Host 'nms.example.org' is known and matches the RSA host key) so why does it add the key for the IP to a user profile?

This is extremely irritating, because when I reinstall a machine, my configuration management system handles collecting and distributing the host keys to all system just fine. But there will be these left over conflicting keys associated with IPs in per-use known_host files that cause warnings on a connection attempt that prevent scripts from connecting.

$ ssh nms -v
OpenSSH_6.0p1 Debian-4+deb7u4, OpenSSL 1.0.1e 11 Feb 2013
...
debug1: Local version string SSH-2.0-OpenSSH_6.0p1 Debian-4+deb7u4
debug1: using hostkeyalias: nms.example.org
...
debug1: Server host key: RSA 6b:5f:b6:e9:13:c3:b7:39:1e:ec:74:05:33:64:4d:5e
debug1: using hostkeyalias: nms.example.org
debug1: Host 'nms.example.org' is known and matches the RSA host key.
debug1: Found key in /etc/ssh/ssh_known_hosts:104
Warning: the RSA host key for 'nms.example.org' differs from the key for the IP address '192.0.2.144'
Offending key for IP in /home/zoredache/.ssh/known_hosts:25
Matching host key in /etc/ssh/ssh_known_hosts:104
Are you sure you want to continue connecting (yes/no)?

How can I prevent ssh from caching this per-IP value in each users known_hosts? Or is there some security reason why I just have to live with this annoying behavior? This is also frustrates me because a couple servers have somewhat dynamic IP addresses. My configuration management handles the DNS updates. But I get these left over per-IP host keys filling up my per-user known_host files.