Previous normal pap smear then HSIL with next one

Hi all, am new to the forum but glad I've found it. Very helpful threads that have given me some mind's peace lately!

My first pap was 3 years ago which came back normal and "white blood cells +". Was told to come back for the next smear in three years time. However, while I was at another clinic for something else I got another another one done 1.5 years ago. Normal result as well. 

My recent smear results registered HSIL so I was referred for a colpo and had lletz done on the spot. Treated piece was sent for biopsy. Currently waiting for results.

Along the way, results were never discussed (I only wondered about the white blood cells in the initial smear result after asking for hard copies of all tests ever done through my usual clinic) and while on hindsight I felt I should've pushed for better dialogue with the consultant who carried out treatment before verbally agreeing while my legs where in stirrups (and better bedside manners, lol), I've comforted myself with the fact that a biopsy would've been necessary anyway. 

My niggling worry is how a normal result could have progressed to a HSIL (= cin2/3?) within 1.5 years. Does this mean the smears may not have picked up anything before or that my immune system basically could not fight against a low grade infection?

Wondering if others have had a similar experience.

Thank you!