Why A frequency counter works by counting input cycles during a precisely defined gate interval, then scaling the count to hertz. That gate interval comes from a crystal oscillator time base, so any error in the time base translates directly, proportionally, into frequency error. This is why counter specs are quoted in parts per million of the time base, and why high-end counters use oven-controlled or GPS-disciplined references.
Watch out The decade dividers and logic are digital, they either count correctly or fail outright, so they do not introduce a small proportional error; the input attenuator only affects sensitivity and level handling, not the count.
A counter is just a stopwatch plus a tally: if the clock is off by 1 ppm, every reading is off by 1 ppm.