[nkmnz]

Their math is 120%-80%=40% while the correct math is (80-120)/120=-33% or (120-80)/80=+50%

It’s more obvious if you take more extreme numbers, say: they estimated to take 99% less time with AI, but it took 99% more time - the difference is not 198%, but 19900%. Suddenly you’re off by two orders of magnitude.