I was taught to knit many years ago by a German friend but just recently picked it back up. Then and now, I struggle with gauging. I've purchased the harmony needles and love them. Unfortunatley they haven't helped me with my gauging. My problem is, I think, I must knit tightly because I just purchased the Garden Cardigan by Jennifer Thompson. I purchased the suggested yarn and needles from knit picks. Here is the problem, I cannot get the gauge with the suggested needles. I would have to cast on very, very loosely causing my work to look very messy. Any suggestions for achieving the gauge without going to a much larger needle?
It looks like someone has basically already answered your question, but I just wanted to reinforce the fact that everyone has a different tension, and what kittenseventoes wrote is very true...the needle size mentioned in any pattern is merely a suggested starting point.
It takes a bit of practice to know what your tension generally is like, but looser knitters can go down in needle size and tighter knitters go up in size. What needle you end up using in the end is really not too important, what is more important is that you are getting the correct amount of stitches per inch and that you like the look/feel of the fabric.
Also, it is good to note here that my tension also changes depending on what I am knitting...for example, my gauge is different depending on if I am using bamboo vs metal needles, or even straight vs circular needles.
Good luck in your project, and if you have any other questions, the KP community is a super resource.