Odd SPC force output for topology optimization analysis

Hi,

I wanted to check the SPC forces in my model and found something strange. In different iterations with nearly the same mass and the same load case (GRAV card) there were different SPC forces.

My load case looks like: (20g in X and Y / 25g in Z; Unit: mm/s^2)

$HMNAME LOADCOL 3'LC_+X+Y+Z'
$HWCOLOR LOADCOL 3 5
$
GRAV 3 01.0 196200.0196200.0245250.0

The *.out file (just a part) looks like:

Design Parameters Summary :
---------------------------

Total # of topology design elements : 258636
Total # of topology cards : 1

Total Volume of Design Material : 3.8620E+06
Total Mass of Design Material : 1.1586E-02
Volume of Non-Design Material : 1.5629E+04
Mass of Non-Design Material : 8.2989E-04

ITERATION 0

Residual energy ratio for static load case 2 = -4.201099E-09
--------------------------------------------------------------------------
Label x-force y-force z-force x-moment y-moment z-moment
--------------------------------------------------------------------------
Sum-App. 4.264E+02 4.264E+02 5.330E+02 -2.312E+05 9.687E+04 1.075E+05
Sum-SPCF -4.264E+02 -4.264E+02 -5.330E+02 2.312E+05 -9.687E+04 -1.075E+05
--------------------------------------------------------------------------

Objective Function (Minimize WCOMP) = 5.03774E+03
Maximum Constraint Violation % = 0.31156E+02
Design Volume Fraction = 3.57023E-01 Mass = 4.96639E-03

ITERATION 1

Residual energy ratio for static load case 2 = -2.439880E-09
--------------------------------------------------------------------------
Label x-force y-force z-force x-moment y-moment z-moment
--------------------------------------------------------------------------
Sum-App. 5.995E+02 5.995E+02 7.494E+02 -2.361E+05 9.898E+04 1.097E+05
Sum-SPCF -5.995E+02 -5.995E+02 -7.494E+02 2.361E+05 -9.898E+04 -1.097E+05
--------------------------------------------------------------------------

Objective Function (Minimize WCOMP) = 4.29319E+03 % change = -14.78
Maximum Constraint Violation % = 0.41841E+02
Design Volume Fraction = 3.57024E-01 Mass = 4.96640E-03

And if i calculate the SPC force for iteration 1 (F=m*a) i get 974 N instead of 599.5 N.
Can anybody explain this?