If there's one thing I've learned in all my years of pattern-writing it's that giving good yardage estimates is a HUGE challenge. No one wants to run out of yarn before they finish their project, and no one wants to be told they need more skeins of potentially expensive and unreturnable yarn than they actually do. So why is it so hard to good estimates? I've come up with my own theories.
1. Stated yardage is not always exact - in fact, I'd argue that it practically never is. It helps to have a scale so that you can verify that your skein weighs what you expect it to. I have found that commercial yarns tend to be the given weight or slightly less (my understanding is that they are allowed a 10% error - they are almost always on the under side) and indie dyers tend to go high. And you still have to assume that the yards/gram given is correct when it may not be. And if it's a thick and think yarn like the one in the photo above, it's just going to be an average at best.
2. Even if you do have a scale and weigh your yarn, how do you know your scale is calibrated correctly?
3. Humidity. Weigh the same skein on a humid day in August and a cold day in January - bet the numbers are different!
4. People use different yardage at the same gauge. Seriously. If 2 of those people had a 3rd person block and then measure their gauge there would be more consistency, but people knit differently, people block differently, people measure differently.
5. Estimating for yardage across different sizes for garments is especially tough - since it's impossible for me to knit all sizes to determine my own yardage, I have to rely on calculations. And if I did take the year that would be required to knit all sizes, my own gauge would vary and the humidity would vary and my scale might get a bit off - so even that would not be 100% accurate. Plus I'd have to knit them all in the exact same yarn - boring!
And the variations can compound - what if I tend to use a bit less yarn for the same amount of knitting than most knitters, weigh my skeins before knitting on a dry day and my leftovers on a damp one (indicating I used less yarn than I actually did), and my scale develops a tendency to weigh on the heavy side (indicating more yarn leftover than actually was) during that time. All of these could mean that another person knitting the exact same thing in the same size and with the same yarn could need much more yarn than the usual 10% cushion that I give. Tester reports can help with this and I do always check their final yardage against my estimates and adjust if necessary. (Throwing out the extremes usually!)
Most designers really do try to give you good yardage estimates but it's just not possible to be 100% accurate! If you plan to use a one-of-a-kind hand dye, buy that extra skein! With commercial yarns that tend to be consistent across dye lots, you may be able to get away with just buying the amount suggested in the pattern. If you're going to play yarn chicken, be prepared to lose sometimes!
