I think Alex Stamos' take on this is pretty good: https://twitter.com/alexstamos/status/1232687398250639361?s=...
I’d imagine this is almost entirely due to scale - Yang probably sends each email to <100k users while Warren/Bloomberg have 25MM+ list sizes.
Typically an email sent to 25 million people is less important than one sent to 100k, and so should be sent to promotions.
I would prefer to send them all to a Politics inbox, to be honest.
It's just an algorithm. Not a big conspiracy. You can definitely influence in which folder you end up. Seems like Buttigieg's team has done a better job at it.
People who want to trash this study with the same old "Have they thought of $obvious_counterargument?" should first browse through their GitHub repo: https://github.com/the-markup/investigation-wheres-my-email
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.