I think a lot of those 'similar' anecdotes stem from the early news stories of aliens and descriptions of them--plus the almost generic shape and size of most of the descriptions...much as a child might draw. Small...no sex organs...simplistic faces...etc.

I believe someone has actually made a timeline of 'alien' images, showing how almost all after a certain point follow the pattern. It's really amazing how much the stereotyped 'aliens' resemble human fetuses, standing up straight with semi-expressions.

It's almost as if no one can imagine other possible configurations now....(well...except Larry Niven and Jerry Pournelle)