That is interesting, and I guess kind of makes sense; each weapon's base damage is a form of limiter on how much power you can pump into it's attack. You can try and stab really really hard with a toothpick, but at the end of the day, it's still a toothpick (horrible analogy, I know). Maybe it's linear in terms of going from that specific class's weakest weapon to Ifrit's weapon (assuming it's the highest damage weapon), explaining why the stats cap the same for the Ifrit weapons with different base damages. I may have to test the ARC theory; this stuff seems pretty complex (and rather awesome lol).
(What parser do you use? And how long do you wait after killing the last mob before stopping it to ensure you have all the data? ^^)