How does a variable get changed when it never gets changed in code

I have a web forms page, where I'm using cookies to pass data between pages.  I pass three values and when I debug in the page load each of the cookie variables has the correct value I expected.   The page load fine and uses several of the variables to load several combo boxes without an issue.  The problem comes when I click the return button to get back to the calling page when I noticed  that one of the  three cookie variables passed into form get changed.  From the cookie variable I set a page level variable(StationID)  using the cookie variable.  I have no idea how either the cookie variable or the page level variable can get changed when there's no place in the code where it gets changed.   I also don't know why only one of the cookie variables get changed or why both the cookie variable and the page level variable get changed.  When I return to the called page the value of the cookie variable is still 0 instead of ...