Now that I look into it a bit more, there appears to be two historic districts, and I mapped the one from the NRHP, and you mapped the one from the Milwaukee government. The Local one is more "real" in that it actually defines preservation guidelines, while the NRHP doesn't define very much, and is (as far as I can tell) "little more than recognition by the government that the resource is worthy of preservation." The Wikipedia article was helpful: https://en.wikipedia.org/wiki/Historic_districts_in_the_United_States