IMO it all has to do with tolerances.
There is going to be some "play" between pink and yellow, those splines need some play to fit.
The torque is supposed to press it all together, make it act like one, but the axle isn't pressing against the face of the hub, that way you would never be sure to get rid of all the play in the bearing.

Not enough lube during assembly, rust, may create the noise when the axle moves a little bit in relation to the hub.

IMO the silicone compound is used to act as a "filler" and a "semi liquid anti squeel ring" between the two.
A light coat of EP grease on the splines wouldn't hurt too IMO.
Where there is grease is no rust

 

I just worked on my axle nuts today. I noticed a "click" when backing out of my driveway (which is inclined going in reverse) and after a complete stop at the top of the driveway moving forward, sometimes it would click. So I researched and read and read and found the "axle nut thread". I pulled off the axle nuts, everything looked clean-some oxidation on the face of the nut that meets up with the hub but nothing unusual. Put some high-temp grease on the face of the nut and retorqued to "new" settings. I do both sides, check everything and clean up. As I am putting my torque wrench away-it is at 240 ft/lbs How could I be such an idiot!
Advice on what to do...leave it as is-do it over again to 220 ft/lbs or is it now back to 181....I don't want to drive the car if it will tear something up.