LTSpice: why does my .MEAS 3dB calculation disagree with the cursors on the plot?
AC analysis produces complex numbers with 'real' and 'imaginary' parts that carry the magnitude and phase information in a vector. The function MAG() is used to extract the magnitude. 

You have done this for vout_max, but not for V(vout), so the comparison will give unexpected results. Change V(vout) to MAG(V(vout)) in the last .MEAS command and it should find the correct frequency.