It’s actually surprisingly hard.

Cut may be the same, pattern may be the same, but a garment will come through as a different size for a variety of reasons including relaxing time (some fabrics need to be relaxed for 24 hours, some for more, after being unrolled), expected size changed due to humidity, expected shrinkage after cutting, fabric relaxation during/after garment manufacture, different dye chemicals, etc.

For example, you could use the same pattern with two different kits but your computer program you use to design the kit tells you that your alternate fabric for the back relaxes to a larger size. You either need cut it smaller and hope it relaxes to the right size, which may happen properly in Turkey where it’s dry but not Thailand where it’s humid, or leave it as-is with the understanding the kit will fit slightly larger than last season’s but everyone who bought a L will still fit an L (rather than some people at the top end washing the shirt and finding the L is skin tight). Better to err on the side of a little big that too small.