To reproduce use this code:
Section section = new Section();
Table table = new Table();
section.Blocks.Add(table);
TableRow row = new TableRow();
TableCell cell = new TableCell();
Paragraph p = new Paragraph();
Span sp = new Span();
sp.Text = "Benefit Start";
sp.UnderlineType = Telerik.WinControls.RichTextBox.UI.UnderlineType.Line;
p.Inlines.Add(sp);
row.Cells.Add(cell);
cell.Blocks.Add(p);
table.Rows.Add(row);
this.radRichTextBox1.Document.Sections.Add(section);

You will see that the text's underlining is wider than it should be.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.