Well, it's not specifically stated to be additive. That's an assumption based on the statement on page 63 of the Super Exciting Guide, that states that fusion via the earrings is multiplicative. The general assumption is that if they have to specify that one is multiplicative, then the other must not be multiplicative. That doesn't necessarily mean that the fusion dance is additive, as I said, that's just an assumption that a lot of people make.
If it is additive, there are other explanations for Gogeta being more powerful than SS3 Goku. My main theory is that what Old Kai said about being rivals making the fusion more complete, (in reference to the earrings) also applies to the fusion dance, and confers some kind of bonus that more than makes up for Goku being restricted to a lower transformed state. This bonus would also apply to Gotenks, as Goten and Trunks were rivals.
My other theory is that the extra energy that Goku would normally use to transform to SS3 is simply being used to increase Gogeta's power level. Both theories could actually be correct, as they're by no means mutually exclusive.