they certainly differ between Unix system variants
they differ between xBSD and Linux.

finding them on Linux is fun:
	errno.h -> bits/errno.h + ifdefs + defs -> linux/errno.h -> asm/errno.h [whew!]

which i note in passing has
	#define EWOULDBLOCK EAGAIN

amongst other values, i find for example on FreeBSD:

#define	ENETDOWN	50		/* Network is down */
#define	ENETUNREACH	51		/* Network is unreachable */
#define	ENETRESET	52		/* Network dropped connection on reset */

but on Linux:

#define	ENETDOWN	100	/* Network is down */
#define	ENETUNREACH	101	/* Network is unreachable */
#define	ENETRESET	102	/* Network dropped connection because of reset */

and on Irix5.3 and Solaris

#define ENETDOWN        127             /* Network is down */
#define ENETUNREACH     128             /* Network is unreachable */
#define ENETRESET       129             /* Network dropped connection because

so all in all it seems a relative waste of time
and effort to make Plan 9's match
one rather than the other.