When Tooltip ContentScrolling property is set to Auto the the tooltip dimensions are not computed properly and the layout looks inconsistent (i.e. usually the left and/or right borders are missing).

Possible workarounds are:

1) using content that will fit in the tooltip and leaving the ContentScrolling property to its default value

OR

2) using some JavaScript to modify the dimensions:
function OnClientResponseEnd()
{
	var activeTooltip = Telerik.Web.UI.RadToolTip.getCurrent();
	if (activeTooltip)
	{
		var contentElement = activeTooltip.get_contentElement();
		if (contentElement)
		{
			contentElement.style.width = parseInt(contentElement.style.width) - 15 + "px";
			contentElement.style.height = parseInt(contentElement.style.height) - 15 + "px";
			contentElement.style.overflow = "auto";
		}
	}
}

where this is attached to the OnClientResponseEnd event of the RadToolTipManager.