If /tp is run on a player, and there's a block on the teleport point, the command will get run twice, and the player will be teleported double the distance. This only occurs when using ~, not when using a precise coordinate. For example, if you are at 0 0 0 and there's a block at 0 0 3 and the command /tp ~ ~ ~3 is run, the player is instead teleported to 0 0 6. This also occurs when executed from a command block as well.