Can’t remove keys from ssh-agent. Even rebooting doesn’t help

A short time ago I noticed that there were three keys in my ssh-agent that I could not delete. ssh-add -l showed three keys; I ran ssh-add -D, and was told "All identities removed."; but then an immediate ssh-add -l showed the same three keys.

If I log out and then back in, the keys are still there. If I reboot the machine, the keys are still there. If I delete the keyring directory in /tmp, I can't connect to ssh-agent any more, but upon logging out and back in, the keys are back. They are invulnerable.

The keys are mine, not anyone else's, as far as I can tell. I can access my usual local services with them. But when I add one of the keys again with ssh-add, giving the path to a private key file, the new key has a different appearance in the output of ssh-add -l:

2048 00:01:02:03:04:05:06:07:08:09:0a:0b:0c:0d:0e:0f /home/jruser/.ssh/jruser-keyname-20110418 (RSA)

vs. the original:

2048 00:01:02:03:04:05:06:07:08:09:0a:0b:0c:0d:0e:0f jruser 04/18/2011 keyname (RSA)

Is there any way to sensibly account for this behavior? I guess there are really two questions:

1. How do the keys managed to be retained even across reboots? My basic knowledge of ssh suggests that keys always have to be added manually.

2. Why does ssh-agent -D lie to me about removing identities?