on logon occurs before the server sends any information identifying itself (although some servers do send the "Looking up your host" "Checking identd" etc. notices before the client sends a USER or NICK command, but often these do not contain a prefix to identify the server, and mIRC does not 'use' these notices to identify the server anyway AFAIK), so mIRC uses the server name you used with the /server command until the server sends numeric 001 (or any other numeric?) identifying itself.
$serverip will help you identify the individual server (just remember the ip address is backwards, eg: 10.11.12.13 is really ip 13.12.11.10) during the on logon event, but this will be kind of useless if your using a bnc or redirect.
