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It is mysterious and perhaps Haddenham can explain. Not the same but might shed a bit of light, I bought a couple of M&S T shirts in a colour and they fitted really well. Exactly the same range I bought a couple of white ones and they were smaller.
Perhaps they are being cut just that bit differently? Maybe the dyeing process is affecting the fabric?
Is you arm/hand near the top in the range of measurements for that size sleeve? If yes, perhaps worth trying the next size up.
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