One weird assumption that is so ingrained is that even fairly liberal people believe that journalists who aren't explicitly conservative (conservative pundits working in "the liberal media" or journalists working for explicitly conservative outlets) are liberalish. There are a lot of reasons for this and I won't go through them all, but one is the general assumption by a certain class of people that other well-educated people who are polite and know which fork to use with the appetizer aren't conservative unless they make it clear that they are. They're thought to be a sort of oddity.
I've seen some people refer to the journalists caught up in the "oh you're friends with the Nazis" scandal as "liberal," even though this is true of most of them only in the sense that they didn't work for explicitly conservative outlets.
I mean, Slate's a bit better than it used to be, generally, but it was only ever liberal (overall, of course there were some genuinely liberal pieces) if you think Will Saletan (not implicated in this) is liberal. And lots of people do! Which is my point.