I can't see that wet cells vary from 105 to 130Ah in the same size group. IIRC a G31 was "always" rated around 100Ah ten years ago, and no one has made astonishing breakthroughs in chemistry, meaning they'd have to thin the case walls down to stuff in 30% more plates.
The Trojan SCS-225 has been around a long time at 130Ah's and the US Battery DCXC 31 about 10 years. Lifeline recently added the 31XT to the line up. It all has to do with the lead, paste grid design etc. etc... US Battery is using the same paste and design they use in their GC batts in the group 31 because they sell them to a huge floor scrubber company who demanded more cycle life.. While a bit more expensive than a typical group 31 they are still less than the Trojan SCS-225.
Group 31 cases are not the only case size to have variances. The 6V GC2 batts range from 200Ah to 240+Ah's all in the same case...
I have capacity tested the SCS-225 and the US DCXC 31 and both meet the Ah rating at least at year one (US SCXC 31) and year two (SCS-225)
Our US Battery/Wal*Mart batteries came within a few Ah's of their 125Ah rating at year 5.....
This is what my 5 year old US Battery group 31's put up during a 20 hour capacity test. They were rated at 125 Ah's:
Battery #1 = 117.7Ah @ 5 years old
Battery #2 = 118.3 Ah @ 5 years old
Battery #3 = 117.9 Ah @ 5 years old