Andyw12345is that because too can mean an excess of something and so you are saying you are not excessively (i.e. overly) sure about something?
Yeah, consider other similar structures, where you have a negation in front of something expressing "completeness", "excess", "large amount" ,etc.
I'm not very hungry.
I'm not so sure about it.
I don't quite understand what you mean.
It doesn't necessarily mean she is going to win.
I'm not too tired. I can go on working.
etc.