Non adequation between 2D and 3D representation

Hi all,

 

I am working on an analytical problem of a circular plate under distributed loading conditions, and with all edge nodes constrained. 

I first modelled the plate with 2D quad elements, thickness 0,35mm.

Then I modelled it with 3D brick elements, thickness 0,35mm (8 elements in the thickness).

 

Of course I would like to find the same order of magnitude concerning both results. But I do not in terms of deflection. 

Does anyone has an answer to this ? 

 

Thank you in advance,

Alice