Reading that document on Macromedia's site gives me light.
http://www.macromedia.com/support/flash/ts/documents/beziers.htm
It says, swf format only supports quadratic beziers, what less accurate than cubic beziers, but easy to render fast. I think, in swf file's beziers are quadratic interpolation of better looking quadratic beziers. Threshold value drives this accuracy (more control nodes, bigger files)