What I'm seeing is layered social media cultures and extensive online experience. Learned informal grammar and punctuation cultivated by the likes of texting, twitter, tumblr, etc. combined with a knowledgeable user base that can also "write properly." These styles are intuitively used as needed depending on the intent of the post or element in the post (effective communication, evocative of an experience, humor, etc.) rather than trying to adhere a site-wide style.
I also speculate the style is influenced by the device used for composing posts: smartphone (more informal) vs. computer (more formal).
case in point, this was a computer post
