Yep, shock horror OP, all DC are different when it comes to immune systems and being prone to different illnesses, even down to individuals in the same family.
My DD9 has always been extremely prone to sickness bugs, went through a period a few years ago (bounce back of germs post-Covid lockdowns) of having one every five bloody weeks for five bloody months. Younger DS7 however hadn't had a sickness bug until last year. That year she was also off with chicken pox followed by Covid, so her attendance was pretty dire.
DS had a pretty tough time of it in nursery (HFM, viral rash, impetigo, cold with temperature, chicken pox), but since he started school (touch wood), his attendance has been around 98-99%.
This year (touch wood!) so far, DS has not had a day off and DD one from illness (stomach ache and feeling sick, though thankfully did not turn into full on sickness bug).
I keep thinking surely we will be hit with something soon though as I've had more Teams calls in the past couple of weeks with other people's ill children in the background than not!
I think one of the reasons is that with more two working parent families, children are more likely to be sent in poorly than 20-30 years ago, increasing spread.
I believe norovirus in particular is also more endemic than it was previously due to new strains (24-25 was a particularly bad year - 5 times higher the rate than average).