A couple of reasons I can think of -
1. He / they think that there are 7 pieces of Voldemort - so that's 6 horcuxes and the one in him (which begs another question - did Pettigrew have to use a horcrux to resurrect him at the end of GOF?)
2. The movie makers made a mistake, if you have read the book then you kind of know what's going on - but at that point in the story they don't - Ron makes a big thing about DD leaving them in the dark - so they have no idea.
Anothe thing which really annoyed me about the film, when they destroy the Horcrux they have to open it first - hang on where did that come from? when they made the unsuccesful attempts to destroy it before they didn't do that!! Just palin stupid.Type your answer here...
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They had to destroy seven horcruxes. Voldemort wanted to divide his soul into seven pieces; since he himself had one of the seven pieces, that would have required only six horcruxes. However, he accidentally created one more horcrux.
There is only seven of them: Diary-Harry Ring-Dumbledore Locket-Ron Cup-Hermione Diadem-Crabbe Harry-Voldermort Nagini-Neville
Voldemort made seven Horcruxes. Harry destroyed one in his second year and Dumbledore destroyed one just before his sixth year which left five Horcruxes to destroy (although he only thought there was four). In the end, every Horcrux was destroyed by different people (Harry, Dumbledore, Ron, Hermione, Crabbe, Voldemort, Neville)
Harry, Ron, and Hermione need to find the 6 (of what they know) Horcruxes and destroy them. they found and destroyed them all to defeat Voldemort.
he only has one