I think
Andrew Beckett in his post already gave the right - but somewhat hidden - answer:
Probably
Cādence marketing people shoved through this
better sounding and more memorable name
virtuoso for
icfb, but - if I remember well - IC6.1.1 didn't provide more
functionality than IC5.1.41 - perhaps with some exceptions like
icca integration and (partly) better tool integration compatibility - same as any new main update as, e.g., from IC4 to IC5 (oh, I know:
Cādence people would protest! ;-) ).