How come its upcoming releases have COS (and GOF later in the year) but not SS

I think they may have got mixed up...they say COS but it shows the SS promo #4 card.
This is exactly the problem, although you will get some interesting information they are not fans so they don't know what they are dealing with. It will amuse you to know that according to their price guide in their current issue the most saught after card in the Prisoner of Azkaban set is a "Whooping Willow card". WHOOP - presumably like my mother it had the whooping cough in it's formative years and this is how it got its name.
Yes many subscribers do say they only subscribe for the promos because by the time they have gone to press their information is unbelievably out of date. I remember our peartree auction was mentioned in their magazine about 5 or 6 months after it ended.
I do agree to a certain extent. However it is ALWAYS nice to read whatever background they might have picked up on the card set, I would expect there to be some pics of base cards and if you are REALLY lucky something more interesting (although I wouldn't count on it because I don't think Artbox had anything to interest them in time). However, let me cite a couple of past examples where their articles have proved interesting.
Their preview of the Prisoner of Azkaban "Classic" set had a picture of the full George Weasley Quidditch Robe, which showed the red robe as we all know it with "Weasley" written in yellow across the back, and the number 5. At the time we had seen plenty of variants with small bits of yellow or one or two variants which were totally yellow, so this provided us with a nice bit of background info.
Another time they had a review of the forthcoming Trilogy (LOTR) set - which revealed no info on costumes but what it did show was a picture of an unsigned Gimli autograph. There was no accompanying text with it or anything and sadly this card never made it into the set which is a huge shame as there are a lot of people who would have liked it. When I see JRD next I shall ask what happened there. Anyway - it was interestig background info