Chris,

It appears to be less due the fact that the note description is not wide enough to force that column to full width.  If you have enough text to force the description to wrap to multiple lines, the width should match very closely.

If you do need the width to match exactly, I'd recommend using the Table format instead.  Mtext will always be a little loosey-goosey with the column widths, while Tables are more able to set an exact width.

--J