Why does meiosis show variation in gametes?

In meiOSis, one source of genetic variation is crossing-over, which is the exchange of corresponding segments of DNA between non-sister chromatids, in other Words chromatids of two different homologous chromosomes. This process recombines segments of maternal and paternal DNA along the molecule. It occurs during prophase I.

Another source is independent assortment of chromosomes. This simply means that when two chromosomes separate and go to opposite poles at anaphase I, the separation of one pair of chromosomes has no influence on the separation of another pair. Even without crossing-over, one individual human could, in theory, produce more than eight million genetically different haploid cells through independent assortment. (It's a bit like dealing a hand of cards. There are many different possible combinations.)