There are two standards for square tapers: JIS [Japanese] and ISO. The main difference between them is that with JIS the square part is 12.73 millimeter at the beginning, and with ISO it is 12.6 *. So a Shimano crank simply won't shove down far enough on a European taper. And a Stronglight will shove down too far on a JIS square taper.

Still, sometimes that is just what you want.

* some sources say 12.63/12.5
** yes, there have been more sizes in use.