Why is my hidden input field renamed after page is rendered?

Question

This is my first post here. Hope you guys can help me.

I have a hidden input field with a runat="server" attribute and with the id and name "scrollPos". When the page is rendered the field name is renamed "ctl00$scrollPos" and the id becomes "ctl00_scrollPos". Why? How can I get rid of this behavior?
The control resides in a masterpage (*.master). I am using Web Dev 2005 Express (.net 2.0).