That's because the plutonium 239 turns into uranium 235
Uh? I'm puzzled as to the process that makes Pu-239 into U-235... Fission of U-235 produces a zoo of isotopes, nearly all of them radioactive. The proportions of each isotope follow something called the M-curve in terms of atomic mass with most of them being roughly half of 235. The commonly-produced longer-lived ones like Cesium-137 (half-life about 30 years) are the ones to worry about since the short-lived ones are gone in a few milliseconds, days or weeks, for example Iodine-131, half-life 8.5 days.
The same thing applies to the fission of Pu-239 nuclei, most products are roughly half of 239 which is damn close but not exactly the same as the fission of U-235. It's one way to characterise a nuclear test, to determine whether it was a uranium or plutonium core (there are other ways and there are methods to obscure the results if the tester doesn't want others to find out easily).
TL;DR -- the Gadget exploded at Trinity in July 1945 released Cs-137 into the atmosphere just like Little Boy did over Hiroshima in early August a month later. Saying that, some Cs-137 probably escaped into the wild from the startup of the Chicago Pile-1 reactor back in December 1942.