An explanation on what the ISO-standard says:
Now remember that these reviews often only cover ONE SAMPLE LENS.
What you get as you buy one might be something else.
Especially in combination with the camera body you have.
My advice is: On any of these sites, find a lens that you OWN… then COMPARE that lens to the others on the site – this will give you a good hint on how the lens you compare to performs, and if you have thoughts about it you also have a reference that can be tried and tested.