it's just rule. i suppose u already know about how count number in korean.
하나, 둘, 셋, 넷, 다섯, 여섯..... when we just count number we use those expressions, but when we use that in sentence they change to 한 개, 두 개, 세 개, 네 개,....or 한 명, 두 명 세 명, 네 명..... like that. well.... it's hard to explain about that for me XD.
anyway u don't have any other choice just memorizing them lol