How Paraview release memory?

Dear Experts,
I found that when create a sphere source then delete it from pipeline browser, the memory is not release for the vtkPolyData that hold all the mesh data.

It may have some potential reasons for this special design?
i wonder why not release memory in destructor of sphere source?
and if this output data also pass to the following filters, then other filter also need release it in destructor?
Since all instance have its reference count, each “Delete” will be OK?

I just checked, deleting a source free the memory. There is small memory overhead that do not get released but this overhead tends to plateau at acceptable value.