Depending on where the rear bracket for the cylinder was mounted, some cylinders have more stroke than the slide has physical distance to move ... thus the space between nut and bracket. I've seen this more on Keystone products.
Do not cinch up the nuts tight to your bracket ... you'll either bust off the interior fascia or the exterior aluminum trim extrusion. Hydraulics are way more powerful than what holds either on the slide itself. Look this over ...
http://www.lci1.com/images/support/lipsheet/0131.pdf