Interesting, factor30. Put 1151 into your converter and it gives MCLI. I think your converter puts the roman numerals into the correct order and then converts. Try putting in a nonsense number like CLIMMMMM. It gives 5151, convert this and it gives MMMMMCLI
Our posts crossed, sir.prize-I was referring to my 1151 answer. Clearly the Roman answer is 151 (so the M isn't part of the Roman numeral).
It's a shame the OP didn't explain everything but maybe it made the challenge more interesting