According to google group's discussion on https://groups.google.com/d/topic/liftweb/MvVYpNbjcDw/discussion

Denis: I am wondered, why lift has both ? and ?? methods. IMHO, more useful and flexible will be to have one method that uses core text resources on bottom. So anybody can override it with own resources and can easily use core strings in his app.
David: I agree with this. Please open a ticket and I'll work through the code and get rid of S.??