Oh, but you see, little Timmy was actually being telepathically controlled by a diabetic alien about to pass out from hypoglycemia, who had him steal the candy bar. Can you prove this was not the case?
This is what reasonable doubt protects against. There will always be the ability to create a (probably fanciful) scenario in which the defendant is not guilty. An unreasonable scenario, one might even say. Being able to convince the jury beyond reasonable doubt is as close to 100% proof as you are reasonably going to ever get.
> Beyond "the shadow of a doubt" is sometimes used interchangeably with beyond reasonable doubt, but this extends beyond the latter, to the extent that it may be considered an impossible standard. The term "reasonable doubt" is therefore used. [Emphasis mine]
If you want to insist that "100% proof" and reasonable doubt are the same, that's of course your choice. I think you'll find that people will tend to disagree with you.
"I doubt that, because a fairy whispered something to me in my sleep."