When using a redstone clock created by placing a command block with the command "setblock ~ ~1 ~ redstone_block 0 destroy" (or ~1 ~ ~, ~-1 ~ ~, ~ ~-1 ~, ~ ~ ~1, ~ ~ ~-1) Command blocks using the execute command which place blocks will act differently depending on the direction. I cannot explain this very well so here is a video.
https://www.youtube.com/watch?v=4oP7LCpC2ug