The-Digital-Picture is a great site, the best out there for thoughtful, careful, unbiased analysis, and a mine of information. But, Bryan Carnathan looks at just one copy (or sometimes 2) of a lens, although he does send some back, and his chart comparisons should not be taken too seriously. For example, one that affected my choice once was the 100-400mm II vs 400mm DO II which has the zoom sharper, which is contrary to the many copy analysis on lensrentals and my own experience.
View the image quality delivered by the Canon EF 100-400mm f/4.5-5.6L IS II USM Lens using ISO 12233 Resolution Chart lab test results. Compare the image quality of this lens with other lenses.
www.the-digital-picture.com
I get asked a lot about different telephoto lenses. Is the 400 DO II better than the 400 DO? Which is better, the Tamron 150-600mm or the Sigma 150-600mm? You've probably noticed that I don't answer those questions. The reason is simple. Our optical bench is designed to test lenses up to 2.5kg...
www.lensrentals.com
I have had the same experience with other lenses, especially EF-M. Your example is not a good choice anyway as comparing a 14-35 with a 15-35 has too many variables other than just a change in maximum aperture.