The basic definition of duty cycle is on_time/(on_time+ Off_time). The on time and off time totally depends upon the rise transition and fall transition.
Due to transition differences, duty cycle changes and hence the calculation becomes bad. Practically, rise and fall transition are not same. In the below picture, we can see that input rise and fall transition are 50%-50% (same) but after travelling through some elements, it varies by almost 8% in both rise and fall.
If we have variation in rise and fall transition from the input transition of rise and fall, then the threshold (50%) will go worse. Hence the pulse width will decrease. If pulse width decreases, then we might lose data which was about to be captured at some time.