Assume you run svn checkout on a repository and receive a working copy at revision 7 (aka, r7) with one file in it called foo.c. You modify the file and commit it successfully. Two things happen: The repository moves to r8 on the server and in your working copy, only the file foo.c moves to r8. The rest of your working copy remains at r7.
$ svn -v status 7 7 nesscg. 8 8 nesscg foo.c $
If you run svn log without any arguments, it prints the log information for the current directory (named "." above). Since the directory itself is still at r7, you do not see the log information for r8.