I grew up in Harpenden. Some of the schools on that list would not have been top in my day!
I think there re a number of reaons why Harpenden schools ranked so highly, both junior and senior.
1) Harpenden has good commuter links to London, so convenient place to live.
2) Pretty looking area
3) harpenden's parents are wealthy ( have you seen the house prices!) and this demographic tends to take an interest intheir children's education, produce more intelligent children,( possibly because have more interest in them), support the schools more, support outside clubs more etc
4) A good reputation can be a self fulfilling prophecy. Ie good school has good children, interested parents which maintains its status, plus people think its 'the ' school to get into so maintains this image also.
5) good schools attract good teachers, have better resources etc
6) Harpenden is bordered by Luton and Dunstable which has average schools, so geographically the options are reduced.
A lot of these points can be made to any popular areas, and not just Harpenden.
Forgot to add, the senior schools produce good academic results, as a result of all of the above?