Express - weird sizing?

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Do you notice at Express (a clothes store - popular in the US), that sizing is often.. off? I'm curious to know if I'm the only one experiencing this. Their Mediums fit me, and I'm usually a LARGE-XL type person. I also can fit into their size 9's pants, && I'm usually a 12!! This is really weird to me. Does this happen to any of you? Any idea why their sizes are so much different than other stores'?