As I understand it, it's just how BitConverter works. It seems that whatever architecture CodeWars uses to run our code happens to be little-endian, and thus GetBytes() acts accordingly. See the documentation:

The order of bytes in the array returned by the GetBytes method depends on whether the computer architecture is little-endian or big-endian.

Do note that the Kata does implicitly describe the first octet being the left-most one (and, correspondingly, the most significant one).

Cheers, that solved it.

It doesn't seem to compile at all in TypeScript. Error TS6053.