The MS Sans Serif font is rendered as Arial in the web environment. The correct one is Sans Serif.

Possible solution is to use the following example setup, the logic of which is to replace the MS Sans Serif font to the Sans Serif one:

<telerik:RadEditor runat="server" ID="RadEditor1" 
OnClientLoad="OnClientLoad">

</telerik:RadEditor>
<script type="text/javascript">
	function OnClientLoad(editor, args) {
		var fontNames = editor.get_fontNames();
		var fontIndex = fontNames.indexOf("MS Sans Serif");
		if (fontIndex >= 0) {
			fontNames[fontIndex] = "Sans Serif";
		}
	}
</script>