I think you're seeing the combination of two factors, of which Jared
has correctly identified one. Try this on for size:
"Most people in the world (and probably most people who use the Web)
don't search but if they do, they use Google"
"Most people in the world don't use social network sites, but if they
do they use Facebook."
(of course these are exaggerations, but play along for a minute please)
What has happened is that a few monopolist-like(*) companies (google,
apple, facebook, amazon, ebay) have dominated their particular segment
of the Web space. They're using non-Web techniques, particularly
smartphone apps, to extend that de facto hegemony. So if you're in
that slice of the Web user population then you are highly likely to
fall into the circles drawn by those companies, to know their UIs and
interaction patterns and to appear to have a certain level of
sophistication merely because of repeated experience and expectations
built up by frequent interaction with a narrow set of UIs.
This hit me recently when I was doing some searching on different
engines to check the SERP of a new site. I'm a heavy Google user for
search, and found myself uncomfortable with the unfamiliar
interactions of Yahoo! and Bing.
What I think Jared is quite correct about is that as insiders to this
sub-slice of Web users it's natural to feel that everyone must be like
us/like that. I doubt you'll find any (many?) people on the IxDA list
who are not themselves pretty sophisticated users of the most common
Web interaction models.
(*) I say monopolist-like in that I don't believe any of these
companies are attempting to create monopolies, but a combination of
factors has rendered them extremely dominant in their spaces.