It's because one twin is right-handed and the other is left-handed (quite common in identical twins, apparently). And so, the way that one knits is a mirror image of the way that the other knits.* When we knit, we tend to twist the yarn slightly at the same time - one twin twists the yarn in the opposite direction to the other. One twists the yarn in the same direction that was spun, and so the yarn becomes more tightly spun, while the other twists it the opposite way. Her yarn becomes slightly unspun as she knits, giving a softer feel to the fabric.
This is not just a theoretical possibility - it happened at Knit Night a couple of weeks ago, and the twins are Ann Kingstone and her sister Marie. They were knitting Ann's latest design, Lanthir Lamath.
|With Ann Kingstone's permission|
To return to the left-handed/right-handed thing: right-handed knitters using the usual British knitting method tend to twist the yarn in the direction that most yarn is spun. Ann is the left-handed twin, so her scarf feels slightly softer than Marie's.
Knitting is just endlessly fascinating.
*PS Not all left-handers knit in the opposite way to right-handers - my left-handed sister, for instance, was taught to knit by our Grandma, who was right-handed, and knits the same way as she did. But it must have been tricky for both of them - Mother had already given up trying to teach her.