Counter square wave generation frequency not accurate

Hi NI community,

  I'm using a usb-6351 daq card generate tunable frequency square wave. I wanted to generate square waves in the range of 10-1000Hz. I'm using the 100kHz time base and toggle mode of the counter to generate the square wave. It seems that it will work fine 1kHz and above, but at lower frequency, it is much less accurate. Is it reasonable?

Best,

Danqing