I want to get the first frequency of a shell (200mm, 150mm,1.5mm,material is Al), its four edges are simply supported, according to thin shell theory , its first frequency is about 246.7Hz. when I use S4R element, the result is very close to theory, but when i try to use C3D8(include C3D8R) element, although my mesh is extremely small(0.3mm size and 15 layers), the result converge to 458Hz. So what is the reason of the difference