I was told by a battery distributor that for wet cells, some companies actually fill with a different electrolyte (different sg) depending on the climate in the area they are sold to. So if you have "New England" batteries, or "Tropical" batteries, even allowing for temperature corrections there is apparently some difference in what the "right" sg should be, depending on how they were initially filled.
If you really want to keep tabs on the batteries, there are also electronic capacity testers now
Some under $50 and that's a third option. If the tester agrees with the rating, you know they're probably just fine.
I just find anytime I mess with testing sg, there's a new hole burned through something shortly after. And if the electrolyte hasn't had time to equalize in the cell, which can take hours, then you need to suck it up, squirt it back, swirl it around...and let it stand to get the air bubbles out too. SG isn't a panacea either.