Because it caches (http://gwan.com/faq#cache) response under high concurrencies. Try to benchmark this code:

  #include "gwan.h" // G-WAN exported functions
  int count=0;
  int main(int argc, char *argv[])
  {
   if (!count) printf("New Counter!\n");
   xbuf_xcat(get_reply(argv), "Hello World %d",count++);
   return 200; // return an HTTP code (200:'OK')
  }