The first machines in the 1850's were all bullet shuttle until Wilson invented the rotary shuttle,which is faster & smoother running than then the bullet style,as to way Singer made them both I'd say that he felt that's all some people would buy since that's what they are used to & then some people are open to the lastest & greatest.
Singer ended up buying out Wison & refined his machine & that is how we know have the 111w ,144w & countless other models.