Indeed, Marek, if you have editors or other controls added/inserted to your treeView, the indexes of the scrollers would be different, i.e you could iterate through your controls collection and check their type to see if the control is a scrollbar.
property is not expected to change while scrolling. This is by design. It is re-calculated when you collapse, expand, remove, add, and insert nodes, but not in the case of scrolling.
In order to help you, it would be best to give us information on your exact scenario.
All the best,
the Telerik team