Can Someone Please Explain to Me . . .

Why it is that after I save my game and exit for the night, with my Sim having 2 days left of an Ailment, he has 3 days left the very next day when I power up again.? I'm really cheesed off about this. This isn't the first time things have gone slightly wonky on me. This is just the latest incident. 

I know I'm not doing this to myself. Let's be clear not all those who get older grow addlepated. Just thought I'd get that one out of the way. 😉

I would just like to know if there's a way to stop this madness.

Please, and thank you, in advance,

~GalacticGal