round(real,integer)

History question:
Why does select round(3,3) work, 
         select round(3.0,3) work, 
but      select round(3.0::real,1) not work?

There's a utility cast in the integer case (described here
http://www.postgresql.org/docs/9.3/static/typeconv-func.html),but not in the real case. 
Is this on purpose, or just an oversight? 
Obviously one can work around it, but I'm sick of doing so, it makes my SQL examples ugly. I’d like to fix it, but not
ifthere’s a reason it’s not supposed to be done. 
P.

--
Paul Ramsey
http://cleverelephant.ca
http://postgis.net