What is the absolute difference that you are seeing if you don't write the results in scientific notation?
Mr. Mather -- I'm not sure I understand your intent, but for the lower case I expected the faces at the ends of the spring to experience equal and opposite Y-directed forces and that those forces be commensurate with the 18 lb preload. What I get instead is -18 (lower face) + 8.63 (upper face) = -9.37, which agrees pretty well (give probably inadequate mesh resolution) with the "Sum Y" and "Resultant" in the table on the lower left.
Does this address your question?
Update: But subsequently increasing the mesh resolution appears to prove me wrong, as the force balance improves slowly with resolution. It seems to require much more resolution than I expected to get reasonable force balance with springs. Sorry to waste you guys' time! -- John Willett