Swinging the Vote?
themarkup.org
themarkup.org
I did and I have gone through the mbox files and I cannot find anything that would explain the curious difference. That is not to say that there isn't a rational explanation, the probably is, but please give the study authors some credit. They aren't dumb.
Typically an email sent to 25 million people is less important than one sent to 100k, and so should be sent to promotions.
Google has clearly lied (based on the blacklists that whistleblowers have released) about their political bias.