Tone from Norway asked me this very good question. She asked if there was any significant difference between Jamieson's and Jamieson & Smith. And if the two companies compete against each other.
First things first. The short answer is: yes.
The longer version is that yes, there's a difference or several.
Jamieson's make yarn from Shetland wool. It's done in a very industrialised manner. They use heavy machinery from start to finish, and their costumers are yarn shops all over the world. Besides making knitting yarn they also machine knit garments sold world-wide. They weave fabric for garments as well, and last but not least, they make blankets.
The Jamieson's knitting yarn is a bit more scabrous, or itching if you like, because the fleece is sorted by a machine. And the machine includes more of the rougher outer layer of the fleece, which is the least soft bits of fleece.
Jamieson & Smith don't have the same machinery, and my guess is it will stay that way. They take pride in making soft, organic, natural shetland wool, and to do that you need to disregard the fact that machines can make things go easier and faster. Due to the extra amount of work put into this type of yarn the price is slightly higher compared to that of the Jamieson's knitting yarns. But well worth it, if you ask me.
Jamieson & Smith's primary products are fleece, knitting yarns along with a small variety of blankets and rugs.
I believe the two companies do compete against each other - as two companies producing Shetland knitting yarn would do - but I think it's not really that big a deal to either of them, since they seem to be doing fine. The whole name confusion is somewhat inconvenient for both of them, I think.
I hope this makes things a bit clearer :)