ok, i think these kids are playing too rough. 20 pts is a big jump and i think adcomms will see that and respect that you weren't feeling up to par when you took the test the first time. even if you could have had the sense to cancel, no one can overlook that much of a score increase. just be truthful about your rationale and i'm sure you'll be fine.
Yeah, I never thought you were an idiot, just without scruples. As you have explained, I'm far more okay with what you've said (not that you need me to validate your actions), although I share some sentiment with RandomGuy. I say wait until you get your score, and if you have done remarkably better, then write that addendum. Being sick the day of the test is a perfectly valid reason for not doing well, and if your score is that much better, than the jump only serves to valorize your argument. In which case, short and sweet is the way to write an addendum. Don't go into the ins and outs of why you were sick. Just say that you did not feel well. You chose not to cancel, but realizing that the score you did get in no way reflects your ability you chose to re-take the test and the newer score is simply far more representative.
It should not be more than a paragraph long. Otherwise you'll just sound like you're whining and making excuses, which may or may not be the case.