Sex appeal is divided into fitness and SPA. SPA treatments can improve sex appeal by 40 points and are for good when done once.
The remaining 60 points can be achieved by doing fitness at the gym. Here you have 4 categories (chest, waist, hips and weight). Each one should be balanced (close to zero) and add 15 points sex appeal. You can increase all of them (in 10 minutes) or decrease one of them (in 5 minutes).
And because this would be a one time task and admin wants some money/CC from you, they introduced the random change. Over time (at least daily) they change the values a little bit (up and down), so you have to keep them in shape or have to pay CC for auto fitness service (this service just keeps the values, it does not increase their sex appeal, which still has to be done by you).
Show the world what kind of pimp you are (lazy or broke) and care for your girls.
Have fun ;-)