Oh wow buckle in because this is an exciting day to encounter a new discovery!

Short answer: Because we have developed some neat tricks to increase the genetic diversity of all our offspring during sexual reproduction!

Medium answer: It’s true that we get half of our genes from mum and half from dad. (More or less.) But only on average do we pass half of our mum’s genes and half of our dad’s genes down to our children. Emphasis on average.

In the real and messy world of biology, genetic recombination is a term we use to describe how we take the genes we got from our mum, and the genes we got from our dad, lightly scramble them, and divide the lightly scrambled genes while making sperm or egg cells. The two big processes are independent assortment of chromosomes and crossing over, which you can look up, but the upshot is that your kids are half you, but not the same half every time.

That’s why if we look at the left of the chart, ‘Father’ is 50% Italian - because ‘Grand Mother’ is 100% Italian. The other 50% of ‘Father’ is a light scrambling of ‘Grand Father’s 50% Irish and 50% Scottish.

The other neat trick is that sperm and egg cells only require half the number of chromosomes of a complete cell. So ‘Grand Father’ could theoretically make a sperm cell that is up to 100% Irish, despite only being 50% Irish himself. And that’s how ‘Father’ can be 36% Irish, which would have required a sperm cell that is 72% Irish and 28% Scottish, despite Grand Father being only 50% Irish himself.

Long answer: all of the above is oversimplified or wrong-around-the-edges or has exceptions and nuances. The more years of formal study I do in genetics the less confident I am about anything!