Bruh the way NIke is inconsistent on sizing and QC on KICKS should be a DEAD giveaway that the same is bound to happen on jerseys, especially when contracted out to random factories in third world countries.
Put it this way. I own about 15 Arsenal Replicas (IE, Not on field authentics) all purchased from either shops, legit ebay sellers, etc, and sizing is ALL OVER the place. Some fit snug, others fit loose, one or two downright baggy. I can only imagine that these different factories, from Vietnam to Indonesia, to Singapore, to Latin America all have different practices in QC and probably aren't as standardized as one would like to believe (and have been led to believe by Nike Inc).
My advice, buy a couple, find the one that fits, sell the extras. Arsenal is a popular squad, and you should have NO problem finding Gooners to sell to.