This thread has been very confusing.
Spitfire, why didn't you link him to tirerack so he could see the pic too?
http://www.tirerack.com/wheels/tech/offset.htm
Basically, a higher offset will be tucked in more. A lower offset will be closer to the fender. Since offset is based on wheel width, 2 wheels with different widths, but the same offset will fit differently on the vehicle.
Here's a couple examples -
17x7 +40 will not be as close to the fender as 17x7 +35.
If you go from a 16x7 +40 to a 17x7.5 +40, the wheel/tire will be 6mm closer to the fender and 6mm closer to the suspension. Clearance decreases evenly on both sides because you only increased width.
If you go from a 16x6.5 +40 to a 17x7.5 +45, the wheel/tire will be 8mm closer to the fender and 18mm closer to the suspension.
Hopefully that helps at least a little.