thats because every time the trade is off the remaining time left, not the original, for example, lets say your time was 100 years, and you have at least 17 years, the rade off is 100-17=83 left, trade for eyes -50% = you have 41,5 years left to live, another trade for eyes is for 20,75 years, another would be for next 10,375 of your years left to you, 4th trade off would be you have left 5 years and around 18 days left of life, with each trade your life span shortens by the half of the ammount you were originally left to live, theoretically you could trade for eternity but at some point you would end up having only a few minutes left ahead of you.