The piecing of the sleeves on the Rogart shirt appears somewhat similar to the sleeve piecing of the
Kragelund tunic (minus the elbow gore), which is Danish in origin. Carbon-dating places it around 1040-1155. I'm adding this to point out that construction methods, especially non-fitted/geometric ones, were relatively common or at least
comparable across multiple European cultures (especially the northern ones) for a bunch of centuries in the central and high Middle Ages. Between ship and foot trade, pilgrimage, warfare, and noble intermarriage, one gets the impression that tailoring methods got around.
As an example, Novgorod's deeper soil layers are littered with the remains of continental trinkets circa 14thc, when the Hansa were cashing in on the fur craze. There is a fascinating woodcut from this time period showing Germen traders and native Russian traders doing business and the only difference in their appearance, IIRC, is the long beards (braided?) of the Russians. Both groups wore continental-style gowns. Unfortunately, the book with this image is now packed, awaiting a move to a new home in a few weeks, so I can't give further detail.
This is just food for thought; not anything one must necessarily apply to Scottish practices.
-Tasha