CellData only available for some cells?

I’m working on a script to output VTK format (.vtu Unstructured Grid Files) of FEM analysis for using with Paraview.

For some result code, I have only the results for some cell, not all. For example, I have 200 cells within my model, but I have only 100 CellData. How we can deal with this situation?
Thank you in advance,