why print screen iphone4 get solution(960px*640px) but make app to set(480px*345px) ?

i have question iphone4 have solution(960px * 640px).

when i make app , i have to set a screen work  (480px * 345px).

i found set a screen work (480px * 345px), it is best fix screen.

not sure you understand.

i need to know, if someone not know can useful.

thank you