Contact pressure and rolling resistance. A smaller tire puts greater pressure on the road, and to a point, increases traction, even in the dirt, and a narrower tire offers less resistance to rolling, thus increasing economy.
When I went to get new tires for this right after purchase, neither the stock size (265/65) or the 265/70 size offered anything above a "B" ply, and since I go down dirt roads all the time and max out the tow rating, I needed a tougher tire, and this was the only all terrain in an 18" near the stock size that there was (255/70-18) I live in a snowy area and the size is great for driving on snow and ice, but they're not great in dirt or rocks since I need to air them up rock hard to get the correct tread contact pattern.
The wider 285 would slip all over the place in snowy weather and decrease economy otherwise, but they would do better in the dirt since I wouldn't need to run 50 lbs per tire. The guys that say the wider tires don't hurt economy probably don't drive 30k miles a year (like I do) and drive hard enough that economy sucks for them no matter what.
So long answer short, the 265/65-18 stock tire only comes in B ply and I drive too much for the 285/65-18 to make sense, so that leaves either the 255/70-18 or now 265/70-18 (which didn't come with "E" rated treads 7 months ago) and I want a slightly taller tire than what's on there so 265/70-18 it is.
