When I worked part time at a fabric store this happened every time I was there::
A fabric shopper would come up to me with her fabric and the pattern she wanted to sew from and ask for fabric for a size 38 (or 42 or whatever).
As always, when I thought the pattern size might be wrong, I asked her if she had measured herself and almost always the customer replied: "No, but I always use size 38 (or 42 etc)".