Cantilever Beam's displacement can't match the analysis result

I have do the simple supported beam's analysis

 

and when I calculate the displacement of middle, it will still have 4.8% difference.

 

But my element size is about 10 mm.

 

Why I can't match them.

 

I watch the z-displacement. 

 

I have refine it but it still wrong.

 

 

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