It is difficult to be certain about this without seeing the wording of the clause in question, but my gut feeling is that the clause probably provides automatic coverage for up to 20% difference over the policy period from the Sums Insured initially agreed without the Insured having to notify the Insurer of additions(/deletions/reductions in value) to the property covered. It seems a little odd for the word "desviación" to be used as this suggests that it applies to reductions in value as well as increases in value, which is not normally the case with such a clause.

The clause MAY allow for an adjustment in premium at the end of the policy period to take account of any changes in the amounts of the property covered during the policy period or the initial premium agreed may already have allowed for this eventuality.

Any changes in values from the Sums Insured agreed between the Insurer and the Insured at inception above 20% would still need to be advised to the Insurer and an addendum agreed.

But, as I mentioned above, this is no more than an educated guess and it is impossible to be precise without sight of the full wording of the clause.

Ref: 20+years' experience in insurance.