"Commands using coordinates (eg. "/tp" or "/spawnpoint") add 0.5 to integers, like usual. To clarify, the number -33 would be interpreted as -32.5, and the number 187 would be interpreted as 187.5."
En gros tu rajoute .5 à chaque coordonnée
Source :
http://www.minecraftwiki.net/wiki/Command_Block#Examples