FIELDSET, LEGEND, HTML, & XHTML

First the question:
Why does HTML validation require a LEGEND for FIELDSET, while XHTML 
validation does not?

Now some background:
While checking my site at Bobby (http://www.cast.org/bobby/) it 
recommended using FIELDSET within FORMs. Now, maybe since I only have 
one field within my FORM this isn't needed, but when I included 
FIELDSET, I could no longer validate as HTML 4.01 Transitional without 
also including LEGEND.

But, another site I checked for reference 
(http://www.neuralust.com/~cac6982/) is written in XHTML 1.0 Strict, 
includes FIELDSET, does not have a LEGEND, and yet it validates.

Is there a discrepency that XHTML does not require LEGEND with FIELDSET 
but HTML does?

Thanks,
Joe.