Patches are often forwarded to Debian. Whether Debian accepts them or not is up to them.
Other times, patches are submitted directly to Ubuntu because it is just before a release and the time to wait for it to get into upstream, request a merge, etc. will overshoot feature freeze or the release date. In these cases, upstream bugs are filed and the patches are handed over. Again, it's up to upstream whether they want to accept them or not.
And then some other times, well, I can't be the only person to think like this. I'm active enough in the Ubuntu community that I think at least a few of the other bugsquad folks would recognize me. Are patches from strangers or recognized names more likely to be accepted anywhere? Ones from recognized names. Who will recognize my name, other Ubuntu members or upstream? Ubuntu. I figure if I get them to approve it, it then gets a "and oh yeah, Ubuntu approved it already" when it goes for upstream review.