When converting CMYK to RGB the document's colour profile is used for the conversion to give a more accurate colour value that is within the gamut of that profile. As PaulMc has said most web based converters don't take any consideration of colour profile into the conversion. I've just taken a document in Illustrator using US Web Coated SWOP v2 with a 0,100,100,0 CMYK value and that gives me an RGB of 227, 6, 19 Doing the same in Designer again using US Web Coated SWOP v2 gives me an RGB of