what's happening is this. seed stitch has your knit and purl stitches alternating every row. when it turns into ribbing, your knit stitches from that row are lining up with the knit stitches of the row below.
you probably gained or lost a stitch somewhere and that is causing your pattern to be slightly off. I would count your stitches.
or what you can do is start starting your rows with a purl stitch instead of a knit stitch.