log --graph --first-parent weirdness

The output of "git log --graph --first-parent" seems weird. Maybe it's because I don't understand everything but here's an example anyway:

$ git log --graph --pretty=oneline --abbrev-commit --decorate

M   2ba1dba... (refs/heads/master) Merge branch 'topic'
|\  
| * 432e062... (refs/heads/topic) Change from branch 'topic'
* | b762236... Change from branch 'master'
|/  
* d7fb80a... Initial commit

Normal --first-parent prints this:

$ git log --first-parent --pretty=oneline --abbrev-commit

2ba1dba... Merge branch 'topic' b762236... Change from branch 'master' d7fb80a... Initial commit

With --graph it goes:

$ git log --graph --first-parent --pretty=oneline --abbrev-commit

M   2ba1dba... Merge branch 'topic'
|\  
* | b762236... Change from branch 'master'
* | d7fb80a... Initial commit
 /

So, it prints the second parent line but it leads to nowhere. Try the same with the git repository and you'll see a _lots_ of parallel branch lines which seem to go nowhere.