I think it's helpful to compare to a widely accepted and objectively testable condition to explain this. Down's Syndrome is testable because it arises from a whole or partial extra chromosome 21. However, the way that the presence of the extra chromosome will be evident is variable. Some individuals will be able to pass GCSEs, work, and own their own home. Some will need support workers checking in on them a couple of times per week. Some will require shared accommodation with daily support workers but can be left overnight. Some will require full time care in a group home. Some will require 2:1 support at all times. Some will be physically robust. Some will have hip dysplasia. Some will have normal heart function, some complications requiring medication, others heart defects requiring surgery. Some will have behavioural challenges, others not. Some will have extensive speech and language therapy needs and others will just have a slightly altered voice because of their enlarged tongue. Some will develop early onset dementia. They all have the same condition.
ASD and ADHD are similar. To get a diagnosis for ASD the criteria are and not 'or'. The subject has to have difficulties in several domains. Their difficulties might be most severe in one domain and relatively mild in another, but they must have difficulties in all of the areas.
ADHD has three hallmarks. Inattention, impulsivity, and hyperactivity. A subject doesn't need to fulfil all 3 areas, but their difficulties must be so significant that it affects daily function, the difficulties must have been present from childhood, and the difficulties must be present in at least two settings. So a child who displays behaviour at school but not at home, or home but not school, would not meet the diagnostic criteria.
Other conditions exist. Some people can have sensory processing disorder. They don't have ASD if they don't also have limits on their social function and restricted patterns of behaviour. Some people can have social communication disorders. They don't have ASD if they don't also have restricted patterns of behaviour.
Liking routine, disliking noise, disliking crowds, etc., aren't, on their own, Autism. It's when those things interfere with an individual's ability to function and they're combined with restricted social-emotional reciprocity, that they are Autism.
This is why brains will look different.