For once Google hasn't been very helpful in turning up similar issues although I'm pretty sure this has been brought up before...
WordPress version: 3.5.1
WordPress SEO version: 1.4.7
Shouldn't the meta description limit be ~140 instead of 156 characters for blog posts?
If Google finds a publish date (where does it get it from? sitemap XML?) it uses up to 15 chars to prefix the meta description by that date (e.g. "May 15, 2013 – "). Also, the snippet preview should be adjusted accordingly.