When changing a combination's "Impact on price" it takes into consideration the VAT that is beset for the initially created product.
When population product databases we often refer to our distributors webpage lists which may sometimes contain only prices with already included VAT. So if the combination's impact (with VAT included) is say 10 EUR, if I enter 10 it's going to apply the product's beset VAT on top of the 10 EUR. In my case I use 25% VAT so it will add 25% on top of what I enter under "impact on price".
In Prestashop one can use either the field Impact on price tax incl. or tax excl. This would be very beneficial in emagic