The frequency of the counter output must be evenly divisible into the frequency of its timebase. The resulting period is further divided by the duty cycle, which defines how long the signal stays high and how long it stays low for each period. If the requested frequency and duty cycle combination can't be produced exactly, then DAQmx will coerce the signal to the closest frequency and duty cycle that is possible. If you want to define exactly how long the signal stays high and low, use ticks instead.