@ksaj @phoe I think ksaj probably meant these actually:
CL-USER> (format t "~o" #xFF)
377
NIL
CL-USER> (format t "~x" #o377)
FF
NIL
CL-USER> (format t "~@r" #o377)
CCLV
NIL

so I'm going to say because ~r is choose-your-own-base radix, with @r being roman numerals and no-choice=words. Not like binary, hex and octal which have special readers as well as format directives.