What this really means is that the designer got gauge with a different needle size... probably. Maybe they knit quite tightly or loosely? IMO the wool company has said on their label that this wool knits up to a nice fabric at x gauge, which is the same gauge, so I'd use it.
The only consideration is the possibility that the designer used a wool knitted at a different gauge from that on the label to get a stiffer denser fabric, or to get a very loose, open, drapey fabric. In that case you knitting the pattern at specified gauge but a different weight wool, could cause problems. I would check the label gauge of the wool recommended in the pattern (use Yarndex?) and get a wool with the same label gauge as the one used in or recommended by the pattern. And knit to the gauge recommended in the pattern. I think that way you can't go wrong.
So: is the label gauge of the wool you want to use the same as the label gauge of the recommended wool?
P.S. are you sure the needle sizes are under the same system, not one in UK size and one in mm or US size?