Why isn't this is own story? sure its a little shorter, but it could just have added a bit more of the friendship, maybe a bit of a backstory of each of the girls and the around 120-150 pages should have easily been filled. 

But this was great as it was too. 

I am always unsure about actually wanting to read those inbetween stories that too many authors are including in their series for the last 10 or so years, since most of them are just completely unnecessary and don't actually tell their own stories or give anything to the overall series. 

This does both in a way. 
It gives its own little story but also gives a better view into the market -which one of the books in the series also has- and its also well written.