Over the years of producing these shirts we have had four different people making them for us (all here in the UK). We now make them in house here in Sussex and have done for the last 2 1/2 years.
It is possible you have shirts from different makers and they will vary if made by different people. Plus if one is a forester, even though the same size, they tend to hang differently and look more tailored.
My point was that the wool they are made from is exactly the same weight, just dyed different colours before weaving. It has always been made on the same loom.