coördinates in game do not work correctly with commands

i have noticed that using coördinates in commands are not accurate. i have tried to place a block using /setblock in a commandblock using exact coördinates but it put the block off by one. also when i used reletive coördinates to teleport me along the y coordinates, it also altered my X and Z coordinates( i dont know by how much)(this could also be because I was standing in a pile of mobs)

With reletive i mean ~ ~64 ~