This is because the HPV vaccine only protects against some HPV types.
There are several other HPV types that can cause cervical cancer that are not protected against by the vaccine, which you may be exposed to through sexual activity.
Additionally, you may have already been exposed to HPV through sexual activity before you had the vaccine. The HPV vaccine can't protect from HPV infections you have already been exposed to.
For these reasons, you can still have an abnormal Pap test result after you have had the vaccine.