Session timeout is quicker than i expecting IN ASP 3.0. Please, check this out

Hi all,
I am using ASP 3.0 for creating a simple web site. My web site's name
is "Greeting" in IIS 5.1.
I write a session for login name: Session("MM_Username").
Pages, have to be restricted from unlogged users, check the
MM_Username session's value in order to know that users logged in. The
session
timeout is 100 minutes for the "Greeting" site in IIS 5.1.

But the problem is: the session is not expired in 100 minutes. It is
expired just in 2 or 3 minutes, it is not explicit and it seems to be
not relative to the timeout in IIS.
Also, I have tried Session.timeout = 100. But there is nothing has
changed.

I did nothing about removing session contents on pages.

Also, I don't know differences between the Session.timeout and the
timeout property of a site in IIS.

Help me, please. What is happening?

.... Orgil