Professionals are possibly more likely to note any awkwardness or flatness as being a sign of autism in autistic people (whereas someone who didn't know about autism might not notice anything particularly 'autistic'). For example, my family are very much used to me and are all quite eccentric/socially awkward in different ways (or used to being around other family members who are), and a question at my assessment was 'do I have an odd gait'. We all said no (we all being my mother, father and myself), but the psychologist put in my notes that I have a 'slightly awkward gait', because she saw me walking, and other people have said that I do have an awkward gait (and a general awkwardness). Other than feeling uncomfortable and uncoordinated when walking sometimes, I had always thought I looked very 'normal', and that did make me a bit self concious for a while (a friend also said that I'm the kind of person she could see getting attacked because my awkwardness suggests vulnerability). Perhaps I am quite lucky in that people seem to accept a lot of my awkwardness without comment, as lots of noticeably 'different' things I do have only been pointed out to me more recently (despite the fact I've always done them).
Also, non-autistic people can read body language, and therefore would notice a lack of body language (including facial expression) where we might not. In someone of average to above average intelligence, lots of people don't tend to notice these things in particular as signs of autism, but a professional would. Also, people who are used to you would accept these things without thought, whereas people who aren't used to you and your ways would be more likely to notice these things. Lots of people think me aggressive/aloof/unfriendly/unapproachable because of my lack of expressiveness, so I sometimes put effort into appearing friendlier, but have also been told that I tend to look more like I'm pulling an unrelated face or I'm grimacing. My family have never reacted to my lack of eye contact, but strangers will react by trying to see what I'm looking at when they're talking to me (as an example of how sometimes we can go years without even realising we do something and seemingly without other people noticing because they're so used to us).
None of these things (that I have mentioned about myself) specifically point to my being autistic, but anyone who knows I'm autistic will likely automatically attribute those things.
Also, I don't think that physically autistic people do have similar features. For example, I remember reading that people with autism have lower ears than people without, but none of the autistic people I know have lower ears than usual. I also think that autistic people are just as likely as the general population to be attractive/unattractive/have defining features/look very average.
If you want to change something (like facial expression) you could probably practise, but if you are happy as you are then I don't think it matters what they have said. It probably doesn't mean that any random person will be able to mark you out as having AS, so I don't think you have too much cause for concern if that is what you're worried about.