interesting... there are two effects that occur at different frequencies. Fa is the parallel resonant frequency which is the frequency that the crystal is a high impedance, and Fr which is the series resonant frequency, where the crystal is a low impedance. if you connect a signal generator across the crystal. and connect the oscope across it as well, there will be a frequency where the voltage peaks, this is Fa, and there will be a frequency where the voltage dips, that is Fr. Fr is lower than Fa