Across federations I've noticed that black belts frankly mean something different to them. I knew of this one school that gave a black belt in two years. The belt basically meant you were a member of the school.
There are Japanese arts in which the first grading you would take would be shodan
. If their grading syllabus says "shodan
requirements = membership of dojo" as the only criterion then nothing would be amiss (and it would be nothing like the minimum requirements being discussed here). It's going to take the average teacher roughly a certain amount of time to pass on certain skills. Even if we were all learning exactly the same thing, shorter timescales to learn that thing mean either training harder and more often under very good teachers or something is amiss and labels are being attached to things that don't reflect their content.
Moreover, we're discussing minimum
requirements. One of the reasons why teachers take a bit longer is to be absolutely sure that they have met those requirements as well as doing whatever extra things their particular lineage regards as important (kata, weapons etc). Some of course are just being competitive, trying to make out that they are better teachers. Eventually it goes beyond merely insuring all the bases have been covered and ends up with teachers trying to fiddle the system to make themselves look good. It can go the other way too with teachers producing high level students in a short time that have not received the skills that are claimed to have been passed on.