basically distortion and flex in the sidewall. If you look at the chart the higher the profile of the tyre the more of a difference you can have between the tread width and rim width. This is due to the increased depth of the sidewall that can take up the difference.
Fitting a wide tyre on a narrow rim can lead to more sidewall flex and if its really missmatched I\'d guess the tyre could fail.
7" wide rims are a little narrow for a 3rd gen. The standard ones are 8" wide IIRC