Why the exact same tensile test, on the exact same specimen, produces two different curves — one that appears to drop before fracture, and one that never does.
Pull a tensile specimen to fracture and plot the data one way, and the stress appears to peak, then fall as the test continues — as if the material were getting weaker right before it breaks. Plot the exact same test data a different way, and the stress climbs continuously, right up to the moment of fracture. Nothing about the physical test changed. What changed is the number sitting in the denominator of the stress calculation — and that single choice is the entire difference between engineering stress and true stress.