Why is it the iPhone 5 when it's the 6th iPhone?

1.iPhone
2.iPhone 3G
3.iPhone 3GS
4.iPhone 4
5.iPhone 4S
6.iPhone 5
Added (1). If the S's don't count as full version releases, it would be the 4th.
>>> Why is it the iPhone 5 when it's the 6th iPhone?