Because we all have 23 pairs of chromosomes, one half from your Dad, one half from your Mum. Therefore, you will have different combinations of genes in your chromosomes from being born. However, when gametes are formed from Meiosis, the chromosomes are split apart, then form a new cell before being split apart yet again. Since 4 cells are created from Meiosis, all 4 gametes will be completely different.