This kind of comment is something that I, and many other autistic people, get confronted with frequently. Not 'looking' or 'seeming' or 'behaving' like one assumes an autistic person should. So I'm going to do a round-up of my thoughts on this. They get more personally pertinent as they go along.
We first started talking about 'autism' in 1938. That means that as a concept it's only been around for 80 years and is still a very new area of study.
Because of a lack of knowledge, effort, and/or sensitivity, the media's portrayal of autism tends to be either nerdy genius males (ie: Dustin Hoffman in Rainman) or non-verbal children, rocking in a corner, 'A Tragedy'. To divide a whole section of society into two camps of stereotypes is ridiculous and unhelpful.
While the media may portray autistic boys and men inaccurately, they simply don't portray autistic girls and women. The first thing I ever saw about autism in girls was a documentary called 'Girls with Autism' on ITV in 2015. I was 25 and was diagnosed shortly afterwards.
There is a theory that autism 'presents differently' in males and females. I don't think it's as clear cut as that, but as a generalisation, I believe it can seem that way because of how we socialise boys and girls differently in our society.
Gender aside, people are individuals. As the saying goes, 'if you've met one person with autism, you've met one person with autism'. If you have an autistic cousin or neighbour, that's great. It doesn't mean I'm the same as them.
This is a biggie for me. 'Masking' in the autism world essentially means acting in a 'neurotypical' manner. To put it another way, I am pretending. I have learnt throughout my life how to 'fit in' and 'be normal(ish)' and defy my natural instincts. It is exhausting. It's also sad because not only do I not get to be myself very much, but it takes years for me to let my guard down and allow someone else to truly get to know me.
When people say, 'you don't look autistic,' what exactly do they mean? I can only assume it's the fault of No. 2 in this list - people are on the look out for Dustin Hoffman. We do not share similar physical characteristics, we do not have to wear a badge, we did not all star in The Graduate.