Because it's a Slavic language and we have our crazy declinations :-) That's why a correct solution would be: "On jí jedno jablko".
Here is a link with all declinations of the word "jeden = one", for all genders: http://cs.wiktionary.org/wiki/jeden
PS: Here it's accusative for a neuter noun.
Good luck with declinations. I would have opted for torture instead myself :D