How to retrieve and change the actual height of Webbrowser DOM element dynamically

I try to change the height of an HtmlElment of a Webbrowser.Document

This is my code:

  // when the element is rendered by browser
  height = elem.OffsetRectangle.Height; //  to get the height e.g 315 
  // here elem.ScrollRectangle height is the same as above

  // setting it 
  IHTMLElement2 dom = elem.DomElement as IHTMLElement2;
  dom.runtimeStyle.posHeight = height; // 315; // or
  dom.runtimeStyle.height = height + "px";
  // here elem.OffsetRectangle height is the same as height: 315
  // but the elem.ScrollRectangle height is more 335!!!

After getting the height when I set the height to the same value (e.g 315), the offsetRectangle height has this value, but the actual height of the element decreases (As I check it visually), the ScrollRectangle height also changes.

Actually there is a mismatch between height I was read and the height I sat. The element has no padding or border.

What is the origin of this behavior? How can I get the height and change it or set it back to the original one with a consistent behavior?