Rankings between arts is not very comparable, we like to pretend that they are, but in reality they are loosely comparable at best. They are often likened to academic degrees, but in academia there are accreditation bodies that provide standard benchmarks of what degrees mean, and it is quite expected from students to go from one school to another and their degrees transfer because they are teaching the same subject. A Ph.D. in Physics from M.I.T. doesn't mean starting over as an undergrad when you go to Cal Tech because they are teaching California Physics instead of Boston Physics.
All a ranking shows is that a martial arts organization has recognized the skill/dedication/development (or to be cynical, pocketbook) of a member and assigned it a number and/or color. Because of similarities in ranking, we get the illusion that they can be easily compared.
If somebody said they were a 9th Dan in Judo, that would mean they were one of the very best judoka alive, and one of the best ever, and they would be famous throughout their entire system. If somebody said they were a 9th dan in Kendo, that's even more outrageous since 9th Dan is even more rare in that art (I don't even know if there are any currently alive). If somebody said they were a 9th Dan in Bujinkan Ninjutsu, that wouldn't be an outrageous claim at all and there are a good number of Shidoshi with that rank or higher.
So, if Sensei Joe down at the local dojo says he's a Nth Dan in whatever art, that isn't inherently an outrageous claim, depending on the context of the ranking system of that art, but it does mean that the wise potential student should investigate the art he teaches and see how it's ranking structure holds up and if his rank can be verified with any larger organization.
The kyu/dan system was created for Judo by Jigoro Kano, and in his own life he never specified it only went to 10, he actually wrote originally there was no theoretical upper limit and that if somebody was good enough there was no reason they couldn't reach 11, or 12 or whatever.
Now, as Kano only promoted others to 10th Dan in his own lifetime, and posthumously he was recognized as a 12th Dan (to my knowledge, he never claimed any Judo rank in his own life, the 12th Dan was given as a sort of posthumous lifetime achievement award to the founder), it became tradition to think of 10th Dan as the limit of human achievement, and when other arts copied the rank system, this went with it.