signed short fd

We have the following definition in fd.c:

typedef struct vfd
{signed short fd;   /* current FD, or VFD_CLOSED if none */...
} Vfd;

but seems we use Vfd.fd as an integer, say in fileNameOpenFile() we have:
vfdP->fd = BasicOpenFile(fileName, fileFlags, fileMode);

So is there any special reason we don't worry that convert an integer to
short will not lose data? Maybe we make the assumption that all OS will
implement "fd" as an array index and is at most 2^16 this big, but why not
use an integer?

Regards,
Qingqing