Why is the output of my forking program different when I pipe its output?

I was looking at some simple code on fork, and decided to try it out for myself. I compiled and then ran it from inside Emacs, and got a different output to that output produced from running it in Bash.

#include <unistd.h>
#include <stdio.h>

int main() {
  if (fork() != 0) {
    printf("%d: X\n", getpid());
  }

  if (fork() != 0) {
    printf("%d: Y\n", getpid());
  }

  printf("%d: Z\n", getpid());
}

I compiled it with gcc, and then ran a.out from inside Emacs, as well as piping it to cat, and grep ., and got this.

2055: X
2055: Y
2055: Z
2055: X
2058: Z
2057: Y
2057: Z
2059: Z

This isn't right. Running it just from Bash I get (which I expected)

2084: X
2084: Y
2084: Z
2085: Y
2085: Z
2087: Z
2086: Z

edit - missed some newlines

What's going on?