Just showing you an example to hopefully clarify that for you.

No Claims Protection guarantees to keep your level of discount the same ie 9 years or (60%)

Example :

Before accident - 9 years NCD (60%) of base premium, lets say £450 giving you a price of £180

Wheras after the accident the base premium has risen the discount has stayed the same. So now you get 9 years (60%) of £600 giving you a price of £240.

NCD protection only keeps the discount from the base premium the same, nothing stops the insurers from raising the base premium. I think I'm a little late in posting but hope that helps =)