It may be the momentum of the liquid distorts the stream shape.
A FANDOM User
It's Coriolis forces. Short spin arm (diameter of the full arc) causes there to be a steeper gradient of acceleration per increment of distance away from the center. Your feet would have noticeably more "weight" than your head. As thigs "fall" away from the center of spin, they move through that accelleration gradient at a discernible rate. You'd have to pour your drink from a few points spinward to have it hit the cup. You can also get weird S-Curves that way, as minor fluctuations get magnified by the daffy angular momentum of a column of fluid.