The sum insured was set to keep the premium down
The average clause pays the same proportion of a loss that the cover bore to the real value. Insure a factory for half what it would cost to rebuild and every claim, large or small, is roughly halved. Nothing about the claim itself is wrong; the arithmetic was settled long before it happened.
An offer that is a clean fraction of what you claimed, with no argument about the facts and no obvious deduction to point at.
We check adequacy before you accept anything, and show the working so the figure can be argued rather than absorbed. Where the cover really is short we say so plainly, because knowing is worth more than hoping, and then we fix it at the renewal so the next loss is not the same conversation.