Yeah, I'm not understanding that either, a 255mm wide tread is a 255mm wide tread (if anything, the tire with the higher aspect ratio will have a wider overall width due to the sidewall bulge that sticks out further).
exactly right, a 255 is 255mm wide no matter what the aspect ratio is. the higher aspect ratio will at a certain point begin to bulge more causing it to actually have a slightly wider footprint