Let's assume the logic that a 'normal' post or thread is 'sorta stupid', with no great ideas, no egregious typos, some wrong or poor and ideas and nothing absurdly insulting. It's the standard for the internet.
The normal internet browser discounts these.
In assessing the intellect of a forum, a browser notes unusual acts of maturity (people accepting that they're wrong is a big one), clever ideas, and exceptionally well written posts. They also see trolls, people with hopelessly flawed ideas who refuse to listen to arguments, spam, technically unnecessary noob hazing, and posts filled with DESU.
Thus, one naturally compares the population of the mid against the latter, whilst ignoring the former-most, garden-variety post. This looks very good for bay12. What they forget is the posts that are boring, meandering, and ultimately thesis-free, which is where the true stupidity is yielded.
tl;dr: People don't notice that which is stupid, but uninteresting, and here at B12 we don't make a shitcan out of it.
Other possibility: People are just shocked not to see trolls, people with hopelessly flawed ideas who refuse to listen to arguments, spam, technically unnecessary noob hazing, and posts filled with DESU around every corner.