Thread counts are somewhat subjective. They represent proprtion rather than specific numbers of threads. Different mills producing cloth of different weights, for different purposes, will want a larger or smaller sett. You can adjust the threadcount to acheive the desired size that you want. (For instance, you'd want a larger sett size for weaving a blanket than you would for cloth to be made into neck ties).

Usually this means multiplying or dividing the numbers evenly, to maintain the proper ratio. However, there is some leeway to this, as you have discovered. In your case, the white pivot line has been doubled, but the black guard lines bordering it have not.

So much of this is dependant upon the weaver's eye. It could have been that if the black lines were doubled, as well, they would have appeared too thick and bulky, so the more narrow count was preserved. In the end what matters is the overall appearance of the tartan.

No one is going to look at the two tartans you have pictured side by side and say, "Well, one is MacGregor, but I don't know what the other is!" No, they are both obviously the MacGregor tartan. On the other hand, if the weaver had changed the black count to "72" for instance, this would have greatly altered the look of the design, to the point where it would no longer be recognizable.

I have a MacGregor kilt myself, that is somewhere around 80 years old. I took a thread count from it once, just to see what it was. I compared it to the recorded MacGregor thread counts I had access to in the Scottish Tartans Society register, and discovered that this kilt matched none of them! However, it was unmistakably the MacGregor tartan, no questions about it.

When I place an order for a custom tartan weave, I often provide the mill with the thread count and then instructions such as "adjust as needed to give a sett between seven and nine inches." Sometimes, too, the weaver will adjest the thread count within the same cloth to allow the selvedge of the cloth to fall at a point in the tartan that is pleasing to the eye for kilting. So you sometimes will get a slightly different thread count in the center of the cloth than you will from the edges, even though visibly you'd never notice any difference just looking at the cloth.

So, the overall look and design is more important than the precise number of threads used. The threadcount is there to give some guidance to the weaver.