Voldemort divided his soul into 8 bits, but created 7 Horcruxes

1. Diary

2. Locket

3. Cup

4. Diadem

5. Nagini

6. Harry (accidental)

7. Ring

The other piece of soul floated around for 13 years after he blew himself up, and was then housed in Voldemort's new body in GoF.

I think Voldemort wanted to created only 6 horcruxes and leave the seventh piece in his body (seven is the most magically powerful number). He couldn't have had seven horcruxes and no soul in his body (he would have been like a Dementor-kiss victim).