BLDC: Zero Cross detection asymmetrical

Hello,

for detecting the Zero-cross of a sensorless BLDC I use a starpoint
reproduction and give this Neutral-Point-signal and the evaluated
Phase to a comparator. At higher speed there is no problem with zero-
cross detection. But at lower speed, the zero crossing detection is
asymmetrical: The falling edge comes late, the rising edge comes very
soon. So the following phases of the running motor are long - short -
long - short ....

Who knows something about this effect?

Best regards Wolf