I think it has to do with the degree of the problem, rather than the problem itself.
Everyone may experience problems with eye contact, but it's the degree of frequency and the level of difficulty that makes the delineation between NT and AC. And because it's a spectrum issue, it's hard to make that dividing line until it's to such a degree that people start taking notice.
For example, it's like that with ADD. Everyone gets absent minded, everyone forgets why they walked into a room, everyone loses track of where they parked their car, and everyone has times when they can't concentrate on the task at hand. But with people with ADD, it happens a lot more often, to the point where it has an impact on their ability to perform on a daily basis.
Plus, in some societies, people can be very nitpicky on how they view what is socially acceptable and what is not. There have been many times when I couldn't figure out what bothered people so much about what I say and do in social situations, but they seem to notice the difference.
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"Not everything that steps out of line, and thus 'abnormal', must necessarily be 'inferior'. " -- Hans Asperger (1938)