Varför har filmer premiär på olika datum?

Här är för filmen Hancock tex.

Release dates for
Hancock (2008)

Country Date
France 16 June 2008 (Paris) (premiere)
Germany 17 June 2008 (Berlin) (premiere)
UK 18 June 2008 (London) (premiere)
Russia 19 June 2008 (Moscow Film Festival)
Canada 1 July 2008
China 2 July 2008
Egypt 2 July 2008
Estonia 2 July 2008
Iceland 2 July 2008
Indonesia 2 July 2008
South Korea 2 July 2008
Thailand 2 July 2008
UK 2 July 2008
USA 2 July 2008
Argentina 3 July 2008
Australia 3 July 2008
Colombia 3 July 2008
Germany 3 July 2008
Hong Kong 3 July 2008
Hungary 3 July 2008
Israel 3 July 2008
Kuwait 3 July 2008
Philippines 3 July 2008
Portugal 3 July 2008
Singapore 3 July 2008
Slovenia 3 July 2008
Austria 4 July 2008
Belgium 4 July 2008 (Wilkinson American Movie Day)
Brazil 4 July 2008
India 4 July 2008
Panama 4 July 2008
Turkey 4 July 2008
Venezuela 4 July 2008
Belgium 9 July 2008
France 9 July 2008
Netherlands 9 July 2008
Russia 10 July 2008
Mexico 11 July 2008
Poland 11 July 2008
Denmark 16 July 2008
Finland 16 July 2008
Norway 18 July 2008
Spain 18 July 2008
Sweden 18 July 2008
Japan 30 August 2008
Italy 12 September 2008
Greece 18 September 2008

Varför har man som sagt olika premiär datum? Vad tycker ni om det? Och varför ihelvete är Sverige så sena?