Ah, I didn't read your node carefully, and failed your comment about zsh.

Well, it's zsh then. From info zsh "Command Execution":

If execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script. /bin/sh is spawned to execute it. If the program is a file beginning with `#!', the remainder of the first line specifies an interpreter for the program. The shell will execute the specified interpreter on operating systems that do not handle this executable format in the kernel.