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

[bdcutler2001]

Here is an interesting one:
cvs up -r HEAD updates you to HEAD of main trunk
cvs 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 would it be possible in future version to add these CVS-
maintained tags (BRANCHHEAD, TRUNKHEAD)?