It has something to do with the way that you put them through your machine. It matters if the seam is going "up" or "down" on the back side.
Watch carefully, it happens quick here.
https://www.youtube.com/watch?v=ymq9Td-z5ew
One more for you which is easier than me explaining the process.
https://www.quiltingroomwithmel.com/...ock-seams.html