I think the sky looks a little grainy perhaps from the Editing work. Im not an expert at all as I dont do much editing (I am poor financialy so no Photoshop for me just yet) but when I have edited I have had this before.
Also if you look at a Shot I put up (look up my post history) for CC of a skyscraper in Hong Kong you will see I had a similar issue with a blue graduated tint I added in after the shooting of the photo. Made it all a bit grainy.....