I think St. Louis’ boundaries are actually fixed by the Missouri constitution. So the low-crime suburbs many big cities have on their outskirts are more absent in St. Louis - they’re part of other cities. That probably contributes to a higher per-capita crime rate.
You are correct. My husband is from St. Louis and the boundaries were set at the time of the Civil War by the Missouri Constitution. That is why the population of St. Louis is only something like 353 thousand, but the metro area with suburbs is 2.8 million. So many times statistics on St. Louis only account for this smaller, poorer section of St. Louis.