What's happening is this:
When you expose stitches at the bottom of your knitting, those loops are not actually lined up with the loops on top, they are half a stitch off. You are picking up what is actually the strand between stitches. Since there were originally 17 stitches, there's only 16 strands between those stitches. You'll have to increase one if you need the same number. And your knitting will be offset by half a stitch no matter what. Not noticable in stockinette, although it will show if you've been working in ribbing, or have other solid vertical lines in the knitting, the line will be interrupted.
Hope that helps!