wrong phasejump from modalfrf?

I have data discribing the oscillation of an two mass swinger, having one spring and one damper each.

I am using matlabs modalfrf to produce amplitude and phase. My code looks like this. acc is the acceleration of one mass.

frf = modalfrf(force,acc,sampling,length(acc));

H = abs(frf);

PhiRadUnwrap =unwrap(atan2(imag(frf),real(frf)));

PhiRad=(atan2(imag(frf),real(frf)));

Does anybody have further knowledge on why my phase doesn't make a phaseshift of pi at the second eigenfrequency but only of pi/2?

Pictures show the plot of Phi in Radians(not using unwrap) over samplepoints and H as representation for the Amplitude over samplepoints. I zoomed in for the second picture so that you can see the second peak at 2100.