I'm so very sorry for your situation VH-MLE.
I'm not a gynaecologist, but I deal with cancer all the time.
1) The normal routine screening Papanicolaou smear does not generally sample cells in the os of the cervix (the canal that leads to the uterus) - perhaps there was malignancy there.
2) Although invasive cancer is often preceeded by dysplastic changes, it doesn't have to be. It is rare, but not uncommon, to see skin cancers arising in areas where there is no background of sun damage or dysplastic change.
3) The world isn't perfect unfortunately. Sometimes minimal dysplastic change can be missed, but remember, dysplastic change is NOT a sure-fire indication that invasive cancer will ensue. Often dysplastic changes regress, so minimal dysplasia is usually treated by repeating the smear a few weeks later.
Life is terribly unfair sometimes.