Why two reactors sized to the exact same average residence time can still deliver completely different conversion. The average is only half the story — the distributionof individual residence times is the other half, and it isn't optional.
It's a natural shortcut: calculate the average residence time a reaction needs, size a vessel — plug flow reactor (PFR) or continuous stirred-tank reactor (CSTR) — to deliver that average, and assume the job is done. For an ideal PFR, that shortcut happens to work, because every fluid element really does spend the same amount of time inside. For a CSTR, it doesn't, because the vessel's defining feature — instantaneous, perfect internal mixing — means individual fluid elements experience wildly different actual residence times even while the average across all of them comes out identical. Matching the average is necessary. It is not sufficient.