Sharepoint masterpage changes while reload

Hello,

I just branded our Sharepoint 2013 Website. I did this without the Design Manager, but with copying, renaming and editing seattle.master + adding a new CSS registration entry in the head area of the master page. So far so good. Everything looks fine.
The only thing what confuses me is, that while my sharepoint page reloads, the css file is not used. While SharePoint says: "Working on it" it still uses the old seattle.master.
I now added the entry to the new css file also to the default seattle.master and everything looks fine. Why does SP behave like that? I googled already but could not find any explanation for this.

thanks,

David