You are exactly right. SLAs do charge slightly over 12 or 6V but right at what you mentioned and quickly drain down to 12 or 6V once the camera is operating. My analog meter shows my Stealth battery slightly over 12V after a charge but as you say, not as easy to read as the digital.
The problem arises when one use a car or golf cart battery at full charge which could go over 1.5V over what the battery is rated and this could fry the cam since the battery amp hours are much higher and they will maintain the higher voltage (13.5V) for a longer period of time. People that use the bigger batteries say they work fine if they draw them down to 12V using a light or trolling motor. Letting them sit for a day or two after charging works too. I might try using a bigger battery but where I put my cams it would kill me carrying them.