I am comparing Approx count Distinct Visitor that is calculate on Customer ID with Unique Visitor. When I compare these two values I expect a Unique Visitor > Approx count Distinct but in some cases, it is smaller. Any suggestion?
Are the number of customer ID crossing the uniques exceeded limit? In that case, it will be counted as 1 item. Also, please note that the function uses the HyperLogLog (HLL) method of approximating distinct counts. It is configured to guarantee the value is within 5% of the actual value 95% of the time. So, it's not 100% accurate always.