There are a lot of leagues (as well as miles). The nautical league is defined as equal to 3 * nm, at least in Spain (were nobody uses it, excpet when reading that Verne's book).
I remember to have been playing with a Casio calculator and a conversion table when I was veeery young. I did a lot of operations, to see if there was any relationship between pi, e, the mass of the electron, the length of a nautical mile, etc. I was very surprised to see that 1851.85
- 3 = 5555.555... I finally figured out that both the mile and the league were related to the mean circunference of the Earth (as well as the meter, of course). I also figured out that pi = 355/113 aprox., and other interesting stuff. Later on, I abandoned this kind of Cabbala and turned to engineering, but that's another story...