The US government doesn’t have standardized sizes required by law(some countries do).
There has been size creep over the years.A pair of size 40 pants today is substantially bigger than 15 years ago.
In the 1960s women knew that for every $10 more they spent on a dress they went down one size.
The reality of clothing production is that time is money. Workers are paid piece work, not a flat hourly wage. A sewing machine operator will sew the wrong size label into a garment rather than wait for a bundle girl to bring the correct ones. One operator leave 1/8″ seam allowance, another 1/4″.
Typically sewing floor inspectors are operators who have become too old and slow to profitably operate machines. Instead they inspect: trim loose threads and look for major defects. They don’t measure garments for size accuracy.
When I left the garment business in 1980, major retailers would spot check one garment per hundred dozen for size within tolerances.