High risk human papillomavirus. Why are there low risk and high risk viruses?

Terminology of risk. We humans apply the high /low designations based on observed behavior. A rattlesnake bite is dangerous a garter snake bite is not, because of individual differences in the snakes. The same goes for HPV in this broad family of dozens of individual strains. Some strains are an inconvenience, triggering a wart in a cosmetically embarrassing site, others can give you cancer and kill you.
Natural variation. There are different strains of hpv virus due to small variations in dna, just like there are different varieties of dogs and other animals or plants.