It's nothing to do with the needles, might be the yarn, or could be how you do your stitches. You might purl looser than you knit, so that could cause a little bit of difference. Also check to see that you have the same number of sts at cast on, bind off. If you can do a couple more posts, you'll be able to post a picture of it, which is really the only way we can see what you do.
sue- knitting heretic