The CY2309NZ is a simple fanout buffer. There is no duty cycle correction circuitry. Therefore, if a non-50% duty cycle input clock is used, the buffer outputs will attempt to follow the input signal, limited only by the frequency and slew rate capabilities of the output driver versus the input. Naturally, there are many cases when the output cannot track the input, e.g. an input pulse << Tr+Tf. In order to ensure the device outputs meet the 40%-60% duty cycle spec, the reference input should have 50% duty cycle.