The answer to this question would be: the HPV vaccine does not replicate in the host
Administering a vaccine will induce the body immune cells to recognize it and produce a specific antibody for that antigen. The vaccine can be beneficial if the titer of the antibody is above a certain level. If the level of antibody is too low, the protection will be low too. Giving vaccine multiple time will increase the titer and increase the protection. HPV vaccine is safe because it won't replicate in the host so it can be administered multiple times.