Abnormal cells CIN1,2 and 3

Hi,
So iv been having hpv positive smears for 3 years now with no abnormal cells. I went for a colposcopy end of July to check this out and biopsies were taken due to them finding mild cell changes. Iv had my biopsy results back which showed CIN 1, CIN2 and CIN3!!! Im booked in for a LLETZ next week but im so confused to why my smear tests werent picking up these abnormal cells? Now im worried they are going to find more.
Please help!!!