From the help file
? for any private message
Unless the NOTICE is coming across in a private message (pm)/query, the ? will not recognize the message. Same thing applies to ON TEXT and ON ACTION events.
Nickserv is a network service, which means that it can be configured to send to the person's status window, rather than a channel or pm/query.
As there isn't a character designed to recognize messages sent via the status window, for these events, the * is a better option, as that way the message is caught no matter where it is actually seen.
