Why do the American and European versions of the Harry Potter cover arts differ?

Also why do some countries have the book title "Harry Potter and the Philosopher's Stone" instead of "Harry Potter and the Sorcerer's Stone"?

This is the cover art of the first book European version:

http://content.answers.com/main/content/wp/en/thumb/c/cb/180px-Harry_Potter_and_the_Philosopher's_Stone.jpg

This is the cover art of the first book American version:

http://www.jcls.org/bookblog/wp-content/uploads/2007/12/n5584.jpg

Just like the cover art (which always differ in other country), there is no specific explanation why the two versions have different titles. Philosopher's Stone is the original name and they just created another title to avoid confusion. The content is the same, so you don't need to worry which version you have to read.