Even the most experienced knitters can have trouble getting gauge. The needle size listed on any yarn label is a very loose guide that acts as a starting place. For any given pattern, you might need a different needle size depending on the stitch pattern, intended drape or stiffness, personal handling of yarn, and a variety of other factors. You should adjust your needle size as necessary until you are able to get gauge—it may take a few tries to land on the right one.
Natural, untreated yarns will knit at a surprising variety of gauges on the same needle size. Conversely, you can get the same gauge with a variety of needle sizes. The reason is not the diameter or the number of yards per 100 grams—that can vary a lot within the same yarn weight. The reason is that different sheep fibers behave differently. Some are relaxed, feel thinner, and make a better knitted fabric on smaller needles. Others are springier, seem to have a larger diameter, and will make a nice fabric on larger needles. It’s important to find the needle size that works best with the yarn.
Likewise, the number of yards contained in 100 grams can vary a lot between fibers—some are heavier and spin to fewer yards per pound and others are light, producing more yards per pound.
You will see a lot more uniqueness between single breed yarns (like 100% Corriedale or BFL) than in the generic wool blends that are commonly used for commercial yarns (100% wool.) Commercial blends use many fiber types together to even out the variations. On the other hand, Bare Naked Wools yarns are purposely spun to celebrate and feature the unique differences between fibers. We only use 2 or 3 fibers in our blends and only one fiber type in our single breed yarns.
All in all, you should strive to get the correct gauge no matter what needle size you require. The most important thing is not the needle size but getting the correct number of stitches and rows per inch. For example, we have a sample of the Highlander cardigan in our Kent DK that was knit on size 7 needles and one in our Stone Soup DK that was knit on size 6 needles—both at the same gauge.