When doing the Rib Stitch, we create each rib with 2 stitches that form a pair. To maintain the pairs or ribs, we need to start on same needles with each row, and that is the first to the third needles. But, what happens when we decrease a stitch at beginning of row; we alter the formation of the pairs, or, we leave only one stitch for the first rib.
So, we must start the knitting by skipping that lonely single stitch, and start with the next rib. So, the first stitch stands alone, and we start the row on needles 2 and 4. This will keep the pairs and the ribs continuing without shifting. The first stitch will be picked up in the return of the circular.
Then, when we do another decrease, we will eliminate the lonely first stitch. So the next pair of stitches becomes the needles 1 and 3. So we will continue the knitting by going back to the original sequence.
What if we increase now. Well, we just added the lonely first stitch back and we would shift back to the 2 and 4 starting for each row.
The important thing here is to compensate for the loss of a part of a rib and maintain all the others in their sequence.