Printing the cover of a book comes with very specific needs for the design. There are a variety of factors that influence the layout of every cover, including the width of the spine and the gutters and bleed. The final page count (which determines the width of the spine) and the trim size of the book are primary factors in the initial setup of the cover.
Below is a simple diagram (based on a CreateSpace template). LightningSource/IngramSpark templates are similar in appearance, but with a few cosmetic differences (blue guide lines for example). An interesting fact to keep in mind is that a CS template and LS/IS template for the exact same book (trim size, page count, etc.) typically have a few small differences that have to be accounted for in the design.