I can't see anything wrong there at all.
All it is showing is that you are learning and understanding the subjects, so you are progressing quicker.
I'd say the time Bristol states is probably a recommended time for those who want to complete within a certain time scale and who are maybe doing it around work etc or for those who may struggle a little more than you are.
As long as you are understanding the subjects and passing the tests, then I don't see any problem with the amount of time it is taking. It could also be an indication that you
might actually be enjoying it so are more engrossed in it than the average person.
I'd just take the recommendation with a pinch of salt as it is really just a guideline, and do as much or as little as you feel comfortable with, and if you have done that weeks allocation of work, simply carry on.
Good luck.