Need realse dates for these games.

Can anyone explain why UK and US have different game date realses?
Assassin's Creed UK 7 Septemeber US 15 June
Resident Evil 5 UK 17 November US 15 June
Saints Row UK 18 May US TBA

And does anyone know the offical realse dates in both countries?
Including;
Ratchet & Clank tools of destruction
Army of Two
Killzone 1 + 2

Sources Play.com, Bestbuy.com

Can anyone explain why UK and US have different game date realses?
Assassin's Creed UK 7 Septemeber US 15 June
Resident Evil 5 UK 17 November US 15 June
Saints Row UK 18 May US TBA

And does anyone know the offical realse dates in both countries?
Including;
Ratchet & Clank tools of destruction
Army of Two
Killzone 1 + 2

Sources Play.com, Bestbuy.com

kier1992