keep in mind that our devices have a fairly sensitive sensor! This means, if move the cap of the device not precisely in the direction of this single axis, other axes may also deliver some values.
For example if you put your SpacePilot a bit askew on your desk and then push the cap exactly away from you, the Cube (for example) might not only become smaller, but also be moved a bit aside.
Anyway, if you have default sensitivity set in the driver the values you're reporting are fairly small and may be a case for warranty replacement of the device.
I've sent you a mail regarding this.