Last Minute Cover Modifications

Past readers of this blog will know that I'm fascinated by book covers. I'm doubly fascinated when a book cover is commissioned, created, released publicly, but then changed before printing. What issue or concern or feeling prompts these changes? Does the author ever have a say in the change? Do publishers have focus groups or something to make this type of decision for them? I know I miss some of these changing covers, but I usually try to save copies when I remember it happening. Here are a couple cover changes I've seen recently:

The Down Home Zombie Blues by Linnea Sinclair -- The red cover is the original, and the blue the second one that it should be released with (I haven't bought my copy yet to find out for sure). The obvious changes are the much larger typeface of the author's name, a more minimized focus on the title (which might seem an odd title to some readers, though not to me), and a much more romance-friendly picture with a heroine who is now not holding a gun.

Mine Till Midnight by Lisa Kleypas -- The white cover is the original and the blue cover is the cover actually released. I think it's just a coincidence, but it is funny to me that both of these covers are now blue. As a library worker, I see a lot of books, and blue is definitely a popular choice, partially because it implies comfort. I really like the change in this particular cover because of the more obvious contrast in color. The picture itself is almost the same (her arms are in different positions, but the dress and the necklace seem unchanged). But the new blue matches the title much better and it doesn't seem washed-out as the first does.