That is typical. Below are measurements of mine.

12.63, 12.6, 12.64, 12.62 (I measured mine four times).

This number is not important. More important number is voltage at load which needs to be over 10v.

If your partner or kids do not close doors properly, it does not matter what batteries you are going to use. If you disenable lights that will help.

Try Diehard. It is a good one.

Best regards,

Alex