Challenged By Layman

Cost!

First let me address why a standard should exist in the first place. The standard gives a minimum insulation required for the application. Cable manufacturers to guarantee that they meet this minimum will typically fabricate to slightly exceed this value so that as production produces standard deviations in the product, all deviations remain above this minimum threshold. This allows manufacturers to be able to produce a standard product line with a cited reference as to how the product should perform.

Now why the standard places this and actually most parameters at a particular number is much more difficult to explain. This is because the parameter is empirically set from failure incidents that have occurred in the past, not from any theoretical design. This is why standards (slowly) change over time, too. A presumption about how much protection is really required in an application can only be verified or refuted by real life testing.

As you would expect, there will be times and circumstances that an application should exceed the standard minimum requirements. Only an experienced designer will know when one should exceed the minimum standards. When these conditions warrant this though the customer should expect to pay a little more.