sorry but my english is very bad, i try to explain.
Some time some user are killed from services for session limit exceded .
This users really don't have other connection to ircd, but on service some session remain active,
example, I make /os userlist *!*@useraffectedsessionip and i see other session, but if i make an whois
on other nick they are not connected to ircd .
Only way to remove this session is a little akill on that host to make session clean ...