My best guess is that it has to do with the audience of the site -- Stack Overflow and more "technically" oriented sites have the dev-focused blog articles featured, while everybody else gets the more meta-oriented blog articles.
Blog posts tagged "bulletin" appear in the sidebar on sites that are considered technology sites (based on the sites listed in the footer) but not on others.
Blog posts tagged "announcements" appear everywhere on the network (except on international sites).
There's a third for "stack-overflow" which only appears on Stack Overflow unless it also has one of the other two tags.
We also have an "international" tag for blog posts written in other languages and that can be paired with a language tag to cause the post to appear on the Russian, Spanish, Portuguese or Japanese SO sites
The bulletin only shows two blog posts at a time and it prefers the most recent. This is why you don't see the "Community working groups updates" post on the technology sites - there are newer blogs that have been featured.