There is a thing called segregation that occurs in dry mixtures of anything. Like materials tend to group themselves as a result of agitating the media. I am in the powdered metal industry where various materials are added to iron powder to give it desired characteristics when it is processed. Mixing is a very technical precise process. Too much mixing increases segregation, too little gives the same result. Mixed batches are tightly packed and shipped on air-ride trailers to reduce the chances for segregation.
The trouble with cereal is that if you pack the boxes too tight, you crush the flakes, too loose and the contents segregate. Your best bet, if you insist on a uniform mix is to either live close to the factory or mix it in a large bowl when you open a new box.