This is something I'm aware long time ago.
When main URL is not set manually, or list of URLs, but RSS feed type of main site, GSA is posting much much less on that project then on the same project that has URL or URLs imported.
Can you please try it yourself:
- make two the same projects
- in first project add several main URLs
You will see that after few days or more 2nd project got much much less submits.