Some differences are directly related to ADHD. Others are the result of secondary characteristics. In special education these are characteristics of a student that result not from the disability but from how the disability plays out in an academic and other settings. For example, a student with a speech impediment may be very timid and anxious in situation in which he or she may need to speak.
In math a major secondary characteristic is math anxiety. This is a performance issue vs an ability issue and it must be addressed as a legitimate obstacle for the student. I work with graduate students who still suffer math anxiety years later.
In the effort to assess student ability performance factors are likely present. It is incumbent upon the educators to mitigate the performance issues to assess true ability.
For example, I conducted an evaluation on a student in middle school who has ADHD. All of her testing records indicated that she would lose focus during the assessment and that the focus was problematic for testing. Before we met I surveyed her on her favorite snack (didn’t know Sour Skittles is a thing), brought this reinforcer along with a bottle of water. She sat through an entire 1 1/2 hour KeyMath Assessment without incident.