SniperOZ's 32M of 311.703s is a 135.85% factor of the reference 423.441s. Multiply that by 0.4 (weight of the benchmark) and you get the 54.34%.
% sign might indeed be confusing, think of it as points: in 32M you get 40 points for matching the ref score and 0.4 points for every percent, for which you improve it.