The device specifications of every device is very important. When you use the device, you can get the specifications from ni.com/manuals.
Click here USB-6341
to get the specifications. In the page 4, the Timing Accuracy Checking the specifications equals to 50ppm. 1ppm is parts per million. So the accuracy equals to 50/1000000 * sample rate. That means when your sampling rate is 10kHz, you may get the frequency of 0.5Hz off. When your sampling rate is 1MHz, the off is 50Hz.