That means either you didn't win at an activity or you didn't complete all the activities with friends. Or you did and didn't save.
Each activity you get 1.66%. So you're missing the Stunt Jumps and 1 Activity. For me it was the strip clubs (I have no interest in the lapdances, sorry R*).
95.83 + 2.5 = 98.33. 98.33 + 1.66 = 100%.