Let me be clear: it's not your lack of mathematical knowledge, quite the opposite, what you want is the correct way to do things - rather it's your mathematical knowledge confusing you! They're fundamentally doing it wrong. I understand why, or at least I have a reasonable hypothesis for why, they are doing it wrong, but they are doing it backwards because they assume most people won't understand why the better number is negative. They should either report that -17% faster/20% slower or +20% more performant/-17% less performant but instead they report +17 faster/ -20 slower.