In theory NT = the brain structure/functioning of the majority of the population.
So if you drew up a big list of brain functionality, and for each found a norm in the population, and average functionality, you would end up with the NT brain.
Very few people would conform to this exactly, but again, in theory, most people would be close enough on enough functions to fall into a boundary of "typical"
ND people would then be those who, on some (more or less) of the functions fall outside of the norm, perhaps some above the norm, but mostly focused on those bellow the norm.
In an ideal world this would be observed as a physical difference in brain function, but in reality its assessed by specialists looking at (sometimes subjective) sets of criteria that weigh a range of factors that the academic/clinical research has justified as being part of an observable pattern in a significant group of people. E.g. Autistic people tend to exhibit X traits, Dyslexic people tend to exhibit Y traits. If you exhibit traits that match the patterns, and are to a severe/impactful enough degree, then = diagnosis is given.
However, the exact groupings of traits, and the exact degree to which it is severe enough to warrant diagnosis are always changing and progressing as our understanding/research into the conditions improve. And the sometimes subjective nature of some of the criteria leaves a lot down to how individual specialists/healthcare systems interpret a person's functionality. So it isn't as concrete or certain as a lot would like it to be. That shouldn't ever be used to undermine diagnosis though - but it should contextualize the framing as "under our best current understanding this is assigned diagnosis".
So yes, it is perfectly possible for a "NT" person to display "ND"-like functionality in some aspects. But they likely don't do so across enough functions and to a severity to conform with any of the patterns that underpin the conditions we currently have diagnosis for.
E.g. A neurotypical person can be just as forgetful as someone with diagnosed ADHD, but if they don't have the consistency of enough of the other traits identified as part of the pattern of ADHD, they are still classed as NT. There is no need to downplay their forgetful-ness, or undermine the other person's diagnosis.
Both should pay their parking fine though....