Thanks. I meant AW10 not 01 - which would make AW10 VS AW09 as 20x9 + 40 offset VS 20x8.5 + 48 offset respectively. However, I can apply what you shared above accordingly. Guess it is just a design tradeoff for weight, milling, etc.There are no rules on which units of measure must be used to communicate offset. It depends on the market or the manufacturer's prerogative—between metric and imperial. And even if units are not indicated in writing, it's pretty easy to tell which system you are looking at. For instance, since offset is measured either outboard (positive offset) or inboard (negative offset) from the center plane of the wheel barrel... it's highly unlikely for a wheel to have 48" of offset; even for a jumbo jet's landing gear.
20" x 9" vs 20" x 8.5"... 20" is wheel diameter. The other number is width of the wheel barrel. All tires have a range of recommended barrel width. And when comparing wheels, the offset (or plane at which the wheel is mounted to the car) is your fixed common reference point. Line this plane up to see where the inboard and outboard rims end up as offset changes.
Because AW01 has 3mm less positive offset, that means the distance from edge of the outboard rim to mounting surface of wheel is 3 mm more than that of AW09. This means the AW01 would poke out 3 mm more than AW09. If both wheels were shod with same brand/model/size tire and inflated equally, the tire on the AW01 would theoretically poke out as much... 3mm. 25.4mm = 1". 3mm equates to 0.11811" (i.e. hardly perceptible).
Cliff notes: Less positive offset means more poke. More negative offset means more poke. This gets a little more complicated when you vary barrel width at the same time, but it's not difficult. Sketching simple diagrams help with visualization. Or, just use this online offset calculator.