vibration of film,model by Structural Mechanics

Recently, I model vibration of film, because I want to compare the Eigenfrequency of 3D and 2D, my model is Structural Mechanics, not Shell or membrane; I find the mesh affects the solution result,
the parameter list:
3D and 2D
L 500[mm] Length width(3D)
thic 0.2[mm] Thickness
T0 100[MPa]*thic Pre-tension force
E1 200[GPa] Young's modulus
rho1 7850[kg/m^3] Density
nu1 0.33 Poisson's ratio

Boundary fixed

the eigenfrequency of solution result of 3D and 2D is following:
2D 3D
f1: 4.322977 Hz f11:7.25292 Hz
f2: 12.093578 Hz f21:14.576149 Hz
f3: 23.74789 Hz f22:21.471876 Hz
f4: 39.27272 Hz
f5:58.67358 Hz

the relation is f11=sqrt(2)*f1; but is wrong, why? Please Somebody may help me, or teache me how to set?

cheers

Brain Lee