True on the framerate -- it's an upper gate. Static video which overburdens a configuration can drop frames to keep up. But interactive presentations need to display each intended frame, and so will drop framerate instead. More here: http://www.kaourantin.net/2006/05/frame-rates-in-flash-playe...

For the "why", that's Adobe Corp's tale to tell, but I haven't yet met a person inside Adobe who wouldn't be very, very happy if Mac/Win performance differences could be made to disappear.