Because of sample variations and test variables:-
https://www.lensrentals.com/blog/2011/10/notes-on-lens-and-camera-variation
So, measurbation tests are good for reference to some extents, but then all those variation factors could come into play and affect the final obtained results.
My own opinion is that for every lens, you should try it yourself long enough to see if you are going to love it or not. Do note that not every performance aspect or characteristic of a lens is measurable. So, things could indeed be subjective after all!
Read Also:-
RiceHigh's Subjective Ratings on Pentax AF Lenses on Pentax DSLRs 2011