Audience counts reported by Website Custom Audiences only measure unique visitors matched to Facebook accounts within the time frame that you choose (up to 180 days). They also exclude people who opt out of ads based on website and/or app usage.

The pixel traffic reported in your pixel dashboard measures the number of times your pixel is fired (even if it's by one person multiple times, or by traffic that didn't come from a person on Facebook) within the time frame that you choose (the last seven days by default).

Because of these differences in measurement, counts may vary between the two.

* Nguồn: Facebook