difference between 2d axisymmetric and 3d model

Hi,

I have a fluid flow model of a perfect cylinder. In 3d simulation I get maximum velocity 0.72m/s whereas in 2d axisymmetric model (rectangle), the maximum velocity is 0.64m/s. I thought both of the model should give same result. Please let me know why I am getting different result. If you need I can send you both the models.

Thank you, Rita