Why does cvs diff -r HEAD diff to branch OR head of trunk?

[BDCutler]

I know this is a repost .. but never heard back on the first post ....

Here is an interesting one:cvs up -r HEAD updates you to HEAD of main 
trunkcvs rdiff -r HEAD ...    rdiffs to HEAD of main trunk

HOWEVER
cvs diff -r HEAD diffs to HEAD of trunk OR head of branch - based on what yor 
sandbox is checked out on.  Not really good if I am on a branch and want to 
compare against HEAD of trunk :-)

Is this a known behaviour?  It does not appear to be documented to work as 
such (actually, docs would lead you to believe what you think it should do - 
diffs to HEAD of trunk ONLY.)

Is there a hidden keyword I am unaware of (BRANCHHEAD, TRUNKHEAD, etc.) or 
some other workaround or would it be possible in future version to add these 
CVS-maintained tags (BRANCHHEAD, TRUNKHEAD)?