Again, it's not that the widest part of the rim needs to be where the tire meets the rim. See the diagram I posted earlier. While a completely smooth transition from the tire to the rim, might be ideal, it's not necessary. The tire is like the leading edge of an airplane wing and the rim is the bulk of the wing. And just as a with a wing, the thickest part of the wing (widest part of the rim) is well back from the leading edge.

I agree completely that the 105% value is just a good target. Each tire and rim combination is likely to have something slightly different that would be the optimum match. And we don't get enough choice about rim width and tire width to be able to hit 105%. All we can do is the best we can to get close to this value. And heck, what is optimum likely changes for wind direction anyway, so it's all just trying to make it as good as we can with what we have to work with.