You have to understand what accurev diff is doing. There are options to diff a file under AccuRev's control against the most recent kept version, the version in the backing stream, or any other version.
In your case, since you just added the file to AccuRev, it's the first real version of the file. There's nothing to diff it against. If you were to make more modifications to that file but nothing else, you could diff against most recent, and it would compare your mods against the version you initially added. If you then keep and promote that file, it exists in the parent (backing stream) now. If you make more local mods, and diff against backed, you can see the local workspace changes against what is in the parent stream.
Hope this answers the question...
Cheers,
~James