You might be wrapping the yarn 'backwards' for the purl stitches, so when you knit into the front leg on the knit row, it twists the stitches. Since you don't purl much in the round they're not twisted. Here's some pictures
that show what twisted stitches are like.
You don't have to change how you purl, just make sure you knit into the back leg on the next row which will keep them straight. If you're not sure about front/back leg, just knit into the leg closest to the tip of the needle.