I also have troubles to see the difference even when I know from authors and psychiatrists that they are different. They even belong to different Axis.
I think they are viewed as similar because they share the fear of the individual to be involved in social situations, groups activities...
My avoidant tendency comes from my social anxiety. But, in spite of my social anxiety, I can have deep relationships with people I feel comfortable with and give me confidence.
I think avoidants cannot have them or find it harder.
Correct me if I'm wrong.