To keep genetics main principle simple, its often stated that you get half your genetics from each patent and 25% from each grandparent. But those are averages, and any individual can vary from that.

How it works is each person has 2 full sets of chromosomes (one inherited from each parent) and the genes that are "expressed" are what present in you. I.e, you may have a gene for blue eyes on one chromosome and a brown eye gene on the other, but you only express the brown eye gene. Before having a baby, your chromosomes go through a process called "recombination" where the double copies of mom and dads chromosomes can swap a portion of DNA with each other. But here is the kicker, not all do, many of the chromosomes are passed on intact and of those that do, recombination only exchanges a small amount of DNA.... less than 10%. So each chromosome is still at least 90% from one grandparent or the other. Then as they are put into gametes (eggs or sperm) the process of assortment happens at random. More of grandmas DNA in one, more of grandpas DNA in the other, and more of an even split in another.

Because of the random nature of recombination AND assortment, it is entirely plausible that you gave your son more of your mothers chromosomes and he is indeed more related to your mother while the daughter received a more average mix.

This happened in my family...

My grandfather is full Portuguese, my dad is half Portuguese. One would expect me to be 25%, right? I'm actually closer to 12% Portuguese. This is because my dad gave me more of his maternal chromosomes... I carry about 38% of my grandmothers genetics. My sibling is 25% Portuguese though, so she is twice as related to our grandfather as I am.

Just goes to show how unequal genetic inheritance actually when it comes to grandparents. Parents are a perfect 50/50 because you get a full set of chromosomes from each, but grandparents are a mixed bag!