Birth date is pretty granular as well. A quick trip to the interwebs gives about 11k-12k people born each day, or 6k of each gender. There are 100k possible ZIP codes. The birth dates aren't evenly distributed, but they're reasonably close. The ZIP codes probably have a lot of clumping, but in dense urban areas they usually start cutting the areas into smaller pieces. The long tail of ZIP codes is probably well-populated. Out of 6k boys and 6k girls that share a birth date, expecting 5k of each to happen to land a unique ZIP out of 100k sounds reasonable to me.