Don't know about all of them. Both of mine are like this--scroll down bottom right -- it has a band width so it is hard to pin down. My big 250AH AGM has the same graph in its specs.
Trojan 6v AGMs have lower voltages per SOCs--see second link below
Both of mine (different brands and sizes) are in the middle of the band when I measured it in real life. I got 13.06 full and 12.4 half full (as seen on Trimetric AH counter)
So 13 vs 12.7 and 12.4 vs 12.1 for a 0.3 diff.
http://www.wegosolar.com/products.php?product=SKR%252d125AGM-Stark-AGM-12V-Solar-Battery-Sealed-125Ahttp://www.trojanbattery.com/pdf/T105-AGM_Reliant_DS_Final.pdf