Its cos you are not limiting the colours , change the no of colours to suit the logo , like if its a red and blue logo , change the colours to rgb or cmyk and limit the no of colours to 2 or 3 (white areas count as a colour) , same can be done with black and white.
If you dont it will see pixelated outlines of the bitmap as a slightly different colour to the object and you will have a trace of the object and the "fuzzy" outline.