The most common reason an STL file may be not quite right is a mechanical connection where some piece isn’t quite as the designer expected, so the 3D model that interfaces with it no longer fits.

[Jouni’s] included his .STL file on his site in case anyone wants to try it out. While he’s designed it for 3D printing, you could probably CNC mill it as well — which would kinda make more ...

Since its creation, the STL file format hasn't changed much and is now considered the standard for 3D printing.