Most weaving mills take some 'liberties' with thread counts and alter the number up or down to make things easier for themselves. Reasons:

1. They want to center a particular stripe in the middle of the cloth
2. The want to get a certain color / stripe to line up along the selvedge
3. They want the size of the sett to be a particular measurement.

Standards for newly registering a tartan (though both of these are over 60 years old) are that it must appear different from every other tartan 'from about 10 feet'. I found this out recently when I enquired about a tartan that was just registered that was almost identical to one of the ones I designed and copyrighted.

If I saw that tartan next to a Ramsay tartan, I'd simply think "oh, that one was woven by a different mill, so the sett came out a bit larger". I would think they were the same thing.

Looking at the STA's website, I can see the thread counts are a bit different and the Ramsay is claimed to have "light grey" instead of white, but every mill I've seen weaves it with white.

IF it were me, I'd get the Ramsay and if anyone asks, tell them it's Ramsay AKA Swan.