The results of the determination of magnetic field strength and flux from weak polarimetric signals in solar inter-network regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes-V line ratio of the Fe I 5247/5250 Å and 15652/15648 Å line pairs is a good indicator of kG magnetic field concentrations, even for magnetic fields with a complex internal structure like those in MHD simulations. On the contrary, the Stokes-V line ratio of the Fe I 6301/6302 Å lines shows no correlation with magnetic field strength. The reason lies in the large difference in the heights of formation of these two lines. The value of the magnetic field strength obtained from the inversion of the Fe I 6301 Å and 6302 Å lines depends crucially on the treatment of gradients of magnetic field, LOS velocity, and temperature even at numerical spatial resolution of 20 km.