Just to be clear, you refer to excess. I assume this is what we call in the states the deductible. For example, you insure a car for $10,000 but if it is totally wrecked or stolen the amount you are paid is minus the deductible amount?
Can't say for charter insurance but for almost all types of insurance I know of, a deductible on the payment is standard. That accomplishes a couple of things for the insurance company.
First it reduces the amount they have to pay on a claim so reduces their costs a little.
The biggest issue is that a deductible usually makes the customer more careful and motivated to
work harder to not use the insurance.
If you could get paid 100% of the value of what you insure then you have zero risk in the business. In that case it makes it much easier for you to cancel the charter or if car insurance, perhaps be less careful about locking the doors or parking in a safe area to reduce risk of theft.
All this reduces the costs to the insurance company and reduces their charge to you. I know with my car insurance the lower I make the deductible the cost goes up a lot.