That's nothing to worry about! Does the camera show f/5.6 when you open the aperture to the max?
You can look into the lens when you set the camera to manual mode, max aperture and a long enough shutter speed to see whether the aperture blades retract fully. You can also try this with the lens detached from the body, just move the little lever on the lens-mount while some light shines into/through the lens.
It might just be that the EXIF-recording of your lens is off...
Thomas (beware: Nikon-fanboy and moderator!) My Pictures
, My Photography Blog