The general rule on PBS is one book one credit regardless if it is a paperback or hardcover, large book or small one. So, why are audio books different? Shouldn't  they also be one credit each instead of 2?  I am just wondering about the reasoning behind this difference.

Not sure, but generally audio books are quite expensive compared to paperbacks or hardbacks.  Granted, hardbacks can be pricey when they first come out, but generally drop dramatically in price after being out a few months.

Melissa is correct and R&R have stated this to be their reasoning behind the difference. Many new people have argued the point and never caused them to sway their position.

Though that reasoning kinda doesn't work when you look at cassette tape books.  Especially 1 or 2 tape books.

The audio CD books all sold quite promptly at the huge rummage sale I work at, and there are rarely any left after the first day at any of the book sales.  But the books on tape just sit there.

And I've seen the same on my shelf.  Books-on-tape just sit there.