In certain situations, the data we receive from Facebook is an estimate of actual activity. We will always label these as such in your Trackers so you can distinguish between actual versus estimated figures.
Facebook calculates unique reach for 1, 7 and 28-day periods, so if you're viewing one of those segments, your reach and per person metrics will be accurate counts.
In some instances you will see estimated data if you are viewing a time period spanning 8-27 days or 29 or more days, and the Tracker report you're viewing includes a unique value like reach and a per person metric (such as likes/person on the engagement summary). Outside of those metrics, all others will be actuals.