Why plus 1 is needed in select system call?

From man select :

int select(int nfds, fd_set *readfds, fd_set *writefds,
           fd_set *exceptfds, struct timeval *timeout);

> nfds should be set to the highest-numbered file descriptor in any of the three sets, plus 1. I'm quite curious about : 1. Why plus 1 is needed, not highest-numbered file descriptor itself? 2. Why request plus 1 operation in user input, instead of handling it inside the system? From sys_generic.c, it seems related __NFDBITS, but I'm not able to go further.

static int max_select_fd(unsigned long n, fd_set_bits *fds)
 339{
 340        unsigned long *open_fds;
 341        unsigned long set;
 342        int max;
 343        struct fdtable *fdt;
 344
 345        /* handle last in-complete long-word first */
 346        set = ~(~0UL files);
 349        open_fds = fdt->open_fds->fds_bits+n;
 350        max = 0;
 351        if (set) {
 352                set &= BITS(fds, n);
 353                if (set) {
 354                        if (!(set & ~*open_fds))
 355                                goto get_max;
 356                        return -EBADF;
 357                }
 358        }
 359        while (n) {
 360                open_fds--;
 361                n--;
 362                set = BITS(fds, n);

Similar topic but not the same: https://unix.stackexchange.com/questions/7742/whats-the-purpose-of-the-first-argument-to-select-system-call