I think when the item level increase was done it was done on a loot table patch prior to 3.1.2 which increased hunter weapon damage by about 20 dps on all weapons 226 ilvl and higher. If you look at Rifle of the Platinum Guard it has 153.7 damage per second at 225 item lvl. Envoy of Mortality has 176.4 damage per second and is 226 ilvl. Thats a 23 dps difference in 1 item level which supports my theory. If you look back you can cross reference this and see.
I couldnt find the changes in the 3.1.2 patch notes but i did find the dev post from Ghostcrawler on May 19th 2009.
"The developers believe hunter dps is too low, so today we are implementing a change to ranged weapon damage to improve dps across the board.
Bows, crossbows and guns of item level 226 or higher (which means weapons from Kel’Thuzad, Ulduar and the Furious Gladiator pieces) will do around 30 more dps. This should translate to a hunter damage increase of a few percent.
We don’t think this will be quite sufficient in boosting hunter dps to where we think it needs to be and we have another buff that we plan to get in sometime over the next couple of weeks. We’ll announce that change when it gets closer.
As hunters may recall, we had an ambitious plan to change the way ammo worked for 3.1 that ended up being overly ambitious, causing us to scale back. However as part of that change, we stopped itemizing bullets and arrows. Since hunters could not upgrade from the ammo available at the launch of Lich King, their damage had started and would continue to fall behind as other classes upgraded their gear. This change basically incorporates the ammo damage into the weapons – as you get better ranged weapons, it’s as if you got better ammo to go along with them. This is likely to be a design we continue for the next few tiers of content until we have the opportunity to overhaul ammo completely."
