I believe it is because dashboard posts are to be seen by friends, and main newsfeed is for everyone. 

I posted in Main newsfeed and the post moved out of main newsfeed and into the dashboard newsfeed. I can't figure out what the main newsfeed is for as it doesn't actually accomplish  anything. I think it's purpose is to confuse the users and the admin. I need to get this stuff figured out before I go public with my site.

See here hope this helps.. 

https://developers.oxwall.com/forum/topic/47525

Thanks, That helps