Goodness & imperfection

The statement "I am 99% a good boy, but ohh that one percent" suggests that you generally consider yourself to be a good person, but there is a small part of you (the remaining 1%) that occasionally behaves in a way that may not be in line with your usual good behavior. It's a humorous or self-aware way of acknowledging that nobody is perfect, and everyone has moments or aspects of themselves that are not entirely virtuous.