@Mammele
It is NOT a hoax.
Unit numbers come from assorted government records.
Way back when, your home may have been part of a subdivision of land and the plot records showed it as unit #4. These would predate the municipality and Post Office assigning street addresses.
this is the Decennial Census, an every 10 year count ordered in Article One of the US Constitution. The Bureaui of the Census conducts many other surveys each year for economic and statistical purposes. Last year they ran a 4 month program trying to verify address accuracy, to make this years’ count be more accurate.