In this case I think it is just the location of your Camera , you need to put the Section Line ( back red handle ) over the centre-line of the Rail , so the dims will snap to the section lines and then ignore the warning ( click ok ) about snapping to them and point markers being added.   ** watch the reach distance on the Dimension, (24" default) as if you go to low , it may also measure the Deck Boards.   M.