Twin tip ski size is deceiving because the tail is extra material added into the equation. Most companies measure the ski as full length of ski(including the tip and tail) while the effect edge is shorter. a 175 may be longer than a 180 depending on the way the company measures. meaning if the 180 is measured tip to tip and not effective edge leaving room for roughly 8-10cm of unused length. therefore the ski is only using 170 - 172 cm of material to make contact with the snow.
Ski measurement is a personal preference as far as what length is good for you.