The frequency accuracy of a frequency counter is determined by:

The fundamental accuracy of a frequency counter relies entirely on its internal time-base generator. This generator acts as the master reference clock against which all incoming signals are compared. If the internal time base drifts due to temperature changes or aging, every measurement it makes will be proportionally inaccurate. High-quality counters often use temperature-compensated or oven-controlled crystal oscillators to maintain this characteristic stability. A very common misunderstanding is that a display with more digits makes the counter more accurate. More digits only provide higher resolution (showing smaller fractions of a hertz), but if the time base is flawed, those extra digits are just displaying precisely wrong information.