Thread counts are relative. This has been discussed in the forum before. For example, the simplest tartan is the red & black MacGregor (aka Rob Roy), which is often expressed as a thread count this way:
K=R

Meaning that the number of black threads is equal to the number of red threads. I could weave the tartan as K10 R10 or K20 R20 or K42 R42 if I wanted to, to achieve a small or large sett. But so long as the proportions stayed the same, it would be the same tartan.

The same is very much true of more complex tartans.

Now some people take this to mean that if you want to change the size of the sett, you must increase or decrease the thread count for each stripe in the tartan by the same ratio. In other words, if you double the count for one stripe, you must double the count for each stripe in the tartan. You double everything. And certainly this would render you the same sett in a larger size.

But is it necessary to adjust the thread count like this in an even manner? No, it is not. So long as the general proportions of the tartan are the same, and the tartan remains visually similar, it would be positively identified as the same tartan.

The black and white Scott tartan you show in your first post is a good example of this. If you compare the computer image with the woven cloth in your kilt, you can see that the sequence of stripes, and the relative width of those stripes, is the same.

The major differences I see are that in the woven example the white pivot stripe on the left is the same size as the stripes on either side of it, whereas in the computer graphic it is half the size (a common occurrence caused by the fact that sometimes a thread count is recorded with a half count on the pivots), and the large white fields are rendered larger in the woven sample. So there are some changes, but the sequence and proportions of the stripes are roughly the same.

Now, let's say instead of adjusting the thread count to make the wide white field (already the largest color block of the tartan) a bit larger, I adjust it to actually decrease its size and instead make the wide black stripe six times as wide. Now the black is the largest color block of the tartan. I have effectively reversed the proportion of black to white. A tartan expert may still be able to look at the design and see that it derives from the Scott B&W tartan, but visually you would have something very different. I would call it a new tartan at that point.

But so long as the sequence and general proportion of the design is upheld, you can change the specific ratios and sizes a bit for personal preference. In fact, I am waiting currently for delivery on a length of custom Armstrong tartan I requested where I changed the thread count to increase the size of the green field, to meet my personal tastes.

If I had the time this morning I could come up with some computer graphics to better illustrate my point, but I hope this helps!