This is probably something I should have explained before; my bad!
When you purchase a copy of my book from Smashwords, you automatically get all the available formats. You can download 1 of them, 2 of them, or all of them. The book also stays in your library on the site where you can download any of the formats again should you lose them (like if your computer dies and that's where you had them) or if you downloaded one version and later have a way to read another so want to download the other version.
In the words of Smashwordsthemselves: "Once a book is purchased, you can download the book in any or all of the available formats, as often as you like."
Just thought I'd explain.
Not too bad of a deal for only $1.99, don't you think?