This is not a bug, it’s rather a limitation in the current implementation. The text box metrics are calculated on the server. The problem comes from the fact that there is no way to convert percentage to pixel or pixel to percentage units on the server. That’s why, all RadTextBox components must be with the same units. This is however possible on the client, because we can get the computed styles of all elements. Here is an example:

<form id="form1" runat="server">
	<telerik:RadScriptManager runat="server" ID="rsm1"></telerik:RadScriptManager>

	<script>
		function onLoad(sender, args) {
			var wrapperEl = sender.get_wrapperElement(),
				inputEl = sender.get_element(),
				labelEl = sender.get_wrapperElement().getElementsByTagName('label')[0],
				wrapperElWidth = parseInt($telerik.getComputedStyle(wrapperEl, "width", 0), 10),
				labelElWidth = parseInt($telerik.getComputedStyle(labelEl, "width", 0),10);
			inputEl.style.width = (wrapperElWidth - labelElWidth) + 'px';
		}
	</script>

	<telerik:RadTextBox runat="server" ID="rtb1" Skin="Silk" Label="This is textbox: " Width="80%"></telerik:RadTextBox>
	<telerik:RadTextBox runat="server" ID="RadTextBox1" Skin="Silk" Label="This is textbox: " Width="80%" LabelWidth="100px">
		<ClientEvents OnLoad="onLoad" />
	</telerik:RadTextBox>
		
</form>
OnLoad client-side event we attach the onLoad event handler in which we retrieve the corresponding html elements and their computed styles.
In your case, the easiest option would be to set label's width in percentage, but if this solution is not convenient you can use the programmatic approach mentioned above.