I can add something to this topic as we are the 'mystery mill' who wove the swatch on the right. In fact we have made it a few times for the clan.
It was made in 12 oz weight to clan specified colours, and thread count, and to answer Matts question...it is Midnight Navy (not black) in ours.
It would seem that when Strathmore reproduced it, they used whatever stock colours they had rather than dye to match...which is perfectly understandable, we do it all the time.
This applies to all tartans...some weaving mills have their own dyeing whereas others buy ready dyed yarn from various sources, therefore there can be significant difference in competing mills versions of a particular tartan.

Gordon
Fraser & Kirkbright Weaving