Using the same rim several 25 labeled tired inflate (under the same pressure) to various widths. It is a bit like comparing shirts. I wear a L, and XL, and XXL. They are all the same size. Unlike shirts I can measure tires with calipers and the standard used is mm, so if the tire says 25 and measures 26 - it is a 26 for comparison purposes. Some hand made tires vary in the same model. As I posted my experience is with tubular tires that are not affected by rim width. That is another variable clinchers introduce. My answer for clinchers is that the same tire is a different width based on how it is mounted and the pressure it is ridden at. But the measurement in mm should override what is stamped on the tire.

The two Vittoria tires below were inflated based on their relative cross sectional area to be "equal" rolling resistance so 130/120 PSI. We did lots of tests - on the road with power meter, HRM, GPS etc and they were close. It came down to the preferred firmness desired, and weight, or a suspension selection. I don't think one size fits all in tire selection.