The resultant nodal deflection that you are looking at, represents the deflection of the node from the initial position. However when the beam deflection is calculated for deflection check, STAAD.Pro follows the process listed next
It first finds out the displaced position of the end nodes of the physical beam segment that is defined by the nodes DJ1 and DJ2.
It joins these two positions by a straight line.
Displacements ( resultant displacement ) at all intermediate sections along the length of the physical member is calculated with respect to that straight line to find out the relative displacements ( say ‘x’ )
Deflection Length ( distance between DJ1 and DJ2 ) divided by ‘x’ gives you the dff.
DFF/dff gives you the ratio.
For your model, the relative displacement comes to around 1.05 inches as seen in the analysis output file ( refer to the attached STAAD.Pro model file with the deflection check incorporated ).
So dff = 26.74x12/1.05 = 305.6
Ratio = 360/305.6 = 1.18 > 1 and hence the deflection check fails.
(Please visit the site to view this file)