sorry... :(

May be
Webserver doesn't send last modified date in http protocol header....

[append]
Me too, I think It make confusing.
As far as I know,
web server send last modified date HEADER.
Not RIGHT "last modifeid date" that human think
or Not the date that page contents were updated if server admin don't deal with it.
Some server are setted to send the current date/time when server make dynamic page as last modified date or browser set current time.