2.0: unexpected modification of a field of user control

[R.A.M.]

Hello,
I wrote a class with private variable which value is modified although
there's no responsible statement, as I see.
Below the code - the class is a user control.

public partial class BooksiDataList : System.Web.UI.UserControl
{
private string ChangedISBN;

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
ChangedISBN = null;
}

protected void BooksiList_Edit(object sender, DataListCommandEventArgs
e)
{
ChangedISBN = ((Label)e.Item.FindControl("BooksListISBN")).Text;
// here ChangedISBN != null
}

protected void BooksList_Update(object sender,
DataListCommandEventArgs e)
{
if (ChangedISBN != null) // HERE PROBLEM: ChangedISBN == null
...
ChangedISBN = null;
}

}

BooksList_Update is called always after BooksList_Edit - which sets
'ChangedISBN'. But in BooksList_Update ChangedISBN is null.
Could you explain me this phenomena?
Thank you very much
/RAM/