Why would locket in 12,GP be real horcrux?

[Mathias Forseti]

No: HPFGUIDX 153170

pforparvati <tareprachi at yahoo.com> wrote:  
>> Most of us think that the locket in Grimmauld Place must be the real 
horcrux. But we know that Harry has seen that locket in Grimmauld Place.
[GoF] Harry has also seen the  original locket around Merope's neck in 
DD's pensieve. If the two lockets would have been same, then surely 
Harry would have recognized it. But he has not. 
I mean as soon as he saw that locket in the Pensieve he would have 
wondered "I have seen this somewhere..."... if not immediately then 
at least after the cave incident...may be after some thinking... <snip> <<

Mathias:
Well, Harry may have forgotton that he saw it, he may remember about 
it in the next book, but 12 GP would be a perfect place for the locket 
with all its security and protection spells.