What happened was I put up the original link a little too hastily -- there were mistakes with the page numbers in the first draft. Unfortunately, while Amazon lets you just update the ebook and considers it the same product, with Blurb, updating it means essentially launching it from go, and the second edition therefore has a different ISBN. The old link that was up was for the edition with the errors, which is no longer for sale. The new link is for the corrected one.
As for the prices, I think blurb's charges are based at least partly on page number (which is why my first book had a slightly higher price than the second, before I discounted it), while Amazon's is more of a flat price.