When price attributes of a product are on a graded scale, a % increase or decrease over base product price makes it more convenient to manage prices of the attributes. This allows a change of base price to reflect proportionally on other attributes.
E.g. base product of 1kg costs 100, attribute 2kg costs 90% higher, thus 190.
When base product price goes up to 500, attribute 2kg would be 0.9 * 500 * 2 = 950
In the current absolute impact terms, we could only add a fixed sum to the base price which is restrictive over a scale of changes.