soit une réduction de cinquante-cinq pour cent

cinquante -- 50
quarante -- 40

Une réduction de cinquante-cinq pour cent -- a reduction of 55%

A reduction from 300 to 135 is a reduction by (absolute) 165. And 165 constitutes 55% of 300 (165 divided by 3). The  value after the reduction (i.e., 135), therefore, is the remainder of 45%. Hence, it is a reduction of 55%.

Ah, sorry, I wasn’t clear. The numbers involved produce a 45% reduction (135/300). It’s not a language question. 

Ahhh, now I see. I thought it had said “reduced by”, but looking back it’s quite clearly “reduced to” (“ qui était descendu à 135€“). Mystery solved - thanks for persevering with your explanation.