u/Background-Baby3694

I recently had my measurements taken by a local tailor to help me more easily buy secondhand clothes online. One that jumped out was a 34' waist. This was something of a surprise for me as a) I'm only 5'7 and 143lbs and b) my best-fitting non-stretch trousers are a 30' waist - that's the actual measurement of the waistband, not the vanity sizing.

How can this be? Is my tailor a rank incompetent? I tried to remeasure myself when I got home by putting a measuring tape around the part of my waist my trousers sit on and it was also 34'. When helping my partner take her measurements recently she had the same issue of apparent waist being much larger than actual trouser waist. I'm assuming I must be measuring wrong but not sure exactly how.