I'm not sure I'm qualified to talk about WHY it flies differently, but I have flown both and I currently fly a Focus II.  The Focus Sport flies very much like the Focus II.  The Venus simply does not fly as sooth as the Focus.  The Focus has a much stiffer fuselage than the Venus which lends itself to more consistency.  It is also has a smaller wing area (780 sq inches vs 866 sq inches) which I would assume makes it a bit more stable in windy conditions.  Beyond that, I'm not sure what else I can tell you.