I thought mine was low (So much so i created a thread on here a few weeks back) as I get around 13.4x, max and with a heavy load it can drop to around 12.8V at idle (As Said). If your worried about the alt to batt cable simply measure the voltage at the alt directly, when I did this the reading I got was as near as makes no difference identical to the reading at batt, alternativly run a jump lead between it and the battery as a test, so mine was fine.

The engine earth strap is the only earth that would effect it but then you would have problems starting, running etc.

As said with everything on you will never get that reading,(Although perfectly possible on modern setups) plus SECS is reading the input at ECU which of course isn't going to be higher than the batt anyway.

Martin