Decided to check out the current peaks and measure the voltage drop on the battery.
The currents FB_L and FB_R are different, because motors have a little difference resistances.
Update 2013.01.04: Motors have the same resistance, the problem is somwhere in the right motor's power line, might be the R_DS(ON) Resistance of the H-bridge, because one of them was prieviously used. Soldering a fresh MC33887 may solve the problem.
Voltage before load 11,74V
Voltage during load 10,37V
So resistance R=(11,74-10,37)/(1,69+1,33)=0,454 Ohm
May be I'll add negative voltage feedback, to compensate the error during loads someday in future.
Love this feature.