To elaborate on this point (because I'm bored and taking a break from knitting a fingering sock weight scarf) here's an example, lets say you're doing a 2x2 rib (K2, p2) scarf. You cast on 32 sts, that would mean your 1st row would be ending with P2 assuming you had started the row with K2, so when you start the 2nd row it would start with K2. Now if you had cast on 34 sts (again starting with K2), your 1st row would end with K2, the 2nd row would start with P2 instead of K2. As you can see (I hope) the number of cast on sts changes how you begin the next row. The pattern also changes which stitch you would make as well.