Session end event is firing immediately after session start

[Ram]

I have a start page for my application default.htm which contains four
frames.

header.htm
login.aspx
rightside.htm
footer.htm

login.aspx conatins username, password fields and login button. When
the login is successful login page writes back javascript
top.location.replace('Homepage.aspx').

Before the Homepage.aspx loads, session_end event is firing. I have
some code in the session_end event which does some processing to
logout the user from my application (i.e., writes some event to
application activity log etc.).

I tried changing default.htm to default.aspx, but no use.

To my surprise, the session variables I set before Session_end fired
still contains values after completing session_end event.

To check whether the Session_end event is firing for the same session
I am logging in, I checked the session id also and it is the same. I
also reset my IIS to make sure that I open only one session.

Can some one please help me to understand why Session_end is firing.