I think you may be incorrect about width not being a factor.
In fact yeah, here are some calculations from another site:
There's more fitment than just offset... If you're running(for example) a 6.5" wide rim with ET 40, it will not clear the same as a 8" wide rim with ET 40..
ET is a hard measurement, not a %age.
For example, how much wheel will overhang towards the body from the mating surface:
6.5"/2 = 3.25" + 1.6"(roughly 40mm) = 4.85"
8"/2 = 4" + 1.6" = 5.6"
and for the section of wheel outwards of the mating surface:
6.5"/2 = 3.25" - 1.6" = 1.65"
8"/2 = 4" - 1.6" = 2.4"
In both cases you have an ET 40 wheel, but the 8" rim will be MUCH closer to the suspension components and body, and it will also poke out more. Just because the offset is the same, doesn't guarantee anything when going wider.
There is a website somewhere with a calculator that shows you all the figures.