vh-mle,
I understand the sensitivity (detection of true positive cases) of pap tests is no greater than 80%, and that is if everything is done properly. So 20% or women with changes will be missed each time.
So, 10000 women with cervical changes are smeared, even with best current practice 20% will be missed which means 2000 will slip through. No problem, as cervical cancer is usually slow to develop.
50% of these changes revert to normal, leaving 1000 women with changes going to the next smear.
Two years later, these 1000 women with changes have smears, 20% are missed (with best practice). 200 women therefore go forward to the next smear with changes, but in the interim 50% revert to normal, leaving 100 ladies.
Smear three and 100 women have changes from the time of the original smear. 20% are missed (with best practices) leaving 20 women with changes which have not been caught for 4 years, and possibly as long as 6. By the time of the next smear 50% of these will have had these changes for a possible 8 years, which is getting worrisome. That's 10 people out of the original 10,000 - only 0.1% which sounds really good for a screening process, but not if you are that 0.1%.
So, although my numbers were chosen to make the maths easier, you can see how it's quite possible for people to be missed by the system even with best practice. Cervical screening depends a lot on the fact that cervical cancer is usually slow to progress, and a lot of changes revert anyway, so you have lots of chances to catch it. It's been incredibly succesful in preventing many early deaths, but it will never prevent them all.
I am sorry for your situation, but it's an unfortunate fact of life that some people are just unlucky.