As you rightly say, Buchanan isn't the only tartan to have many variations upon a theme, it's one amongst many.
Over the years various setts have been woven , some incorrectly by weavers, but still called Buchanan, some by weaver who chose to make subtle changes to make the sett easier to weave, or nicer to their eyes, and then of course some authorities might have given the wrong thread count either by misuderstanding or simply a typo error.

With all tartans the colours are listed simply as blue, red, yellow, green etc, and it is up to weaver to choose which actual tones/hues to use. It is possibly this bit that allows/offers the greatest variation in availability between weavers.
Add to this the ancient/weathered/reproduction/muted/modern variations and you simply increase the actual number permutations.
Some weavers do a smaller or a larger sett variation as well, this is often useful when the tartan has a very large sett in it's own right, or when the tartan might be woven in a different yarn as well.
Of course each tartan does have it's own thread count as per a register, and that should be followed, but that formality is relatively recent.
I'm sorry that not much of what I've said gives you a definitive answer, but it's my usual response when asked the same question by my clients,and I suggest to them that they pick one that they like, and would be happy to wear.

If we think back to when names were given to tartans , oft times , a design was picked from a trade catalogue , and then named, without much of a background history, it is the name of that design that is the important thing , not the actual colourings, however varied they might be!

Good luck with your quest, Buchanan is one of my family tartans so it is certainly of interst to me as wel!