The “estimated reach” is an estimation of the users that will receive the push campaign, not the real computed value as its purpose is to give you a quick result right after creating your campaign.
We take a small representation of all your token base and estimate the number of users that match the query then give you an approximate result.
It performs best on significant userbase (e.g: 15 M tokens and higher) when queries match a critical amount of users, but it can produce an unexpected result when you’re trying to target few users: 4 users out of 15M users for example.
To be more specific: on userbase containing more than 64K tokens, targeting filters are applied on a random 64K sample in order to give you an approximative reach.