why are double books like eargon/eldest or sue grafton 3 in 1 books worth more then one credit?

Because they're still only a single bound volume with a single ISBN.

Right, It's still a single book, albeit with two stories. You wouldn't expect a book of short stories, like "The Illustrated Man" to be worth ten credits  because it has ten stories.

I think it is this way because it is simple. There are other sites with various point systems or allow the user to specify the points.  If you don't feel that you are getting the "value" you would like for them here, maybe try one of the other websites.

So far I like this website better than the others because it is simpler, easier to use and has a lot more books.  But you may prefer the other websites for some of the options they allow.