maximum displacement for a circular plate calculated from ansys differ with the theoretical value

[winterfell99]

For a circular plate(20mm diameter and 2mm thickness) with clamped edge and a load of 50000Pa applied to the front face; The maximum displacement calculated using Ansys differ from the maximum displacement calculated using the formula w(r)=pa^4/64D*(1-r^2/a^2).
I used the default settings in Ansys mechanical to generate a simple mesh.

PS I am just starting to learn Ansys and FEM. I'd really appreciate any help.