I was so confused by this too... and here is how i figured it out.
When you do a knit stitch, it puts a little purl on the back. When you do a purl stitch, it puts the purl on the front.
So... when you turn it around, the stitch you just knitted, become a purl, which you then purl into to keep the bumps on the same side.
Does that make sense?