The most common accommodation for testing is extra time and a quiet room. But of course not every 504 requires extra time.
It is difficult because there is no diagnostic test for some conditions covered by 504s such as ADD. It is diagnosed by questionnaire and it is not that hard to figure out which answers would lead to an ADD diagnosis (which is a real, potentially debilitating condition). It is on a spectrum so where is the line between needing accommodations and doing OK. Is being able to stay on task something that is measured by the standardized tests? Should the kid with the high IQ and high attention score higher than the kid with the same IQ but low attention.?
Anxiety as well. What is the point at which being anxious is normal and when it is a disability (believe, I have a kid with anxiety so I know it when I see it, but his was not related to testing). Parents with means can tip the scale to a diagnosis that leads to extra time. or other accommodations.
The tests use speed as one aspect of scoring, presumably the student with the faster processing and better understanding of the material will finish more quickly and have higher accuracy.