Because the edge is in ribbing. Ribbing creates columns of knits and purls by knitting the knits and purling the purls as they appear. A knit stitch on the RS will look like a purl on the WS. When you work that stitch on the WS you purl it.
Seed, or moss, stitch does the exact opposite. You have to knit the purls and purl the knits as they appear.
Now, why your pattern says P ONE when you are working 2x2 ribbing I'm not sure. Does it have a selvedge stitch on one end? That, or is it a misprint?