ABAQUS/Standard - 2D beam elements vs 3D shell elements

Dear collegues,

I am modeling deformation of a simple triangular shape, /\, which is constrained at one end and the other end is free to move on the x direction. There is a displacement loading on the top corner and the output is the reaction force on the y direction at the free end.

 I used Abaqus/ Standard and modeled this problem in 2D beam elements and 3D shell elements. The material is aluminum with plasticity and nonlinear geometry is on. 

 My question is that why is there a difference in the results when I compare 2D beam to 3D shell element results? If I remove plasticity the results are same. Is plasticity handled in different ways in shell elements versus beam elements? (2D and 3D beam elements give the same results, no problem there)

Any idea for a possible reason is welcomed.

Thanks

Bilim