How can the random distribution of alleles result in a predictable ratio

Let's recall Mendel's law of independent assortment that states, "During the formation of gametes, alleles of genes assort independently into each gamete"

How can the random distribution of alleles result in a predictable ratio?

For example, We have a trait for flower color either yellow or white, and the alleles for the flower color are Y and y. The yellow flower color is a dominant trait, and be expressed if plant has YY or Yy genotype. We cross a plant with genotype Yy with another plant Yy.

P1                                                   Yy   x  Yy

F1                                               YY : Yy: Yy: yy

Phenotype                        Yellow: yellow: yellow: white

Genotypic ratio                 1           :               2       :                 1

Phenotypic ratio                               3         :          1          

So, we see that phenotypic ratio is different from genotypic ratio. Although the distribution of alleles is random but still there are only four possible outcomes for a trait. Hence, we can predict the phenotypic and genotypic ratio of offspring by drawing a punnet square or gamete formation.