SN74HC14N

I have a question about SN74HC14N. In one of my design I need to have a few nanosecond delay on one signal. The signal has 50% duty cycle, 162ns high and 162ns low. I put two gates of SN74HC14CN in series and the signal as input to them. The output must be the same as input with a few nanosecond delay because of propagation delay. I see here the output signal has delay respect to input signal but the duty cycle is not anymore 50%. The duty cycle is around 49.39%, 162ns high and 166ns low. Based on data sheet tPLH , tPHL are the same as tpd and Tr , Tf are the same as Tt, therefore the output after two gates in series must be the same as input but is not. I did the test with Vcc=5VDC and Ta=18c. Please let me know how can I get the same duty cycle for output from SN74HC14.